Yotsuba&!/WMG

Yotsuba's an alien, possibly related to Narue
This explains the green hair and lack of familiarity with common items. She spent a lot of time star-watching on the starship she came here on, and assumes (in the Yotsuba & Stars chapter) that the brightest star in the sky is Earth because that's what it used to be when she was going here.
 * Then the last chapter would be about her real parents coming back to get her. ;_;
 * ...But then Yotsuba decides, in the end, to stay on Earth, because, after all, "Earth is best after all, right?".

Yotsuba is a war orphan from a third world country like Bosnia or Croatia.
This is why she is "invincible": she has seen hell, so everything else is heaven. It also explains her constant fear of the "enemy".
 * This... seems reasonable. Remember that when she first saw fireworks, she thought that they were under attack.
 * Possibly an autocratic or communist country, as she is very impressed by a large department store: "Did the State build it?"
 * Perhaps she's from a place like Malaysia or Indonesia; when her eyes are closed, she shows something of an epithelial fold. Though Mrs. Ayase's suggestion of Hawaii would explain Yotsuba's occasional use of Gratuitous English.
 * As if girls raised in Japan need an excuse for Gratuitous English. Yotsuba absorbs pop culture like a sponge, and she clearly watches a lot of television.

Other origin theories

 * Well, that's true enough (though it's not clear how much time she's spent in Japan, if she still remember the island "to the left"). And if we're going to rationalize her Gratuitous English, we also need to rationalize her gratuitous Spanish.
 * Gratuitous Spanish might be because Kiyohiko Azuma Did Not Do the Research; she is, in fact, a dekasegi Brazilian orphan. Brazilians never give up; even the powers said so.
 * Alternatively, she might have been Cuban. It would explain both the gratuitous English and the Gratuitous Spanish, and it is an island that's a long way left from Japan. Or maybe Colombian, if you need a place where they'd speak Spanish and where she'd have developed a fear of explosions and stuff.
 * Thats unlikely. In Colombia the term enemy is almost never used, much less so by civilians. Its just Paramilitary or Guerilla. so she would have asked "is that the guerilla?". Also, unlikely she is from Cuba cause. 1st, there are no explosions there, 2nd, Cubans are kinda good at stooping foreigners from adopting their child's. He could have adopted legally, but that would imply that he met her in an orphanage, otherwise, no way he could have got her out of Cuba without having to smuggle her out. I am guessing she is Nicaraguan or something.
 * This troper recalls seeing the first page of a Yotsuba doujinshi that chronicles how she met mister Koiwai. He was working in the peace corps in a ruined village when a dirty, impoverished, and orphaned Yotsuba approached him nervously. I wanted to find the rest of it ever since because it looked incredibly good.
 * Report Shit has seen it as well, but I do believe that it is a stand-alone image rather than the part of something.
 * Where exactly can one find this image? It sounds interesting.
 * A theory of mine is that Yotsuba actually comes from Ireland. Her hair is shaped like a four leaf clover, her name means "clover", she uses Gratuitous English, and her adopted father is a translator. The languages highest in demand in Japan are Chinese and English. Her father may have spent time as a Japanese translator in Ireland, one of the richest countries in Europe, and one with a high foreigner-to-native ratio.
 * Irish people don't have green hair. O_o
 * Yotsuba does. This is because...

Yotsuba is a leprechaun.
See above. She acts as if she is invincible because she is a magical creature and therefore is invincible. She changes the world around her in subtle ways, probably subconsciously, making it a benign and friendly place to live. She doesn't show any other characteristics of being a leprechaun because she has been raised by a human almost her whole life.
 * Her "pot of gold" is the Yotsubox where she keeps her treasured possessions.
 * OTOH, why would a leprechaun be afraid of tengu, as she was in a recent chapter?
 * Tengu are also magical, supernatural creatures that are more combat-oriented than leprechauns and are capable of genuinely hurting her; her fear is similar to the fear a mouse has for a cat.
 * On a map, Ireland is an island to the left of Japan.
 * All the leprechauns I've ever seen depicted have the stereotypical Irish red hair. Which is not to say that green is impossible...

Yotsuba is a changeling.
The spell to disguise her as a human baby wasn't cast properly, leaving her with green hair. Her birth parents were really freaked out about her because of this, and essentially abandoned her. (Which is entirely compatible with several other WMGs.) Now, to support this, she's obsessed with festivals, is never seriously injured, even when it appears she will be, and claims to have moved from "the left", a direction associated with the Fae.

Yotsuba is Alpha.
v.0.1. Mr. Koiwai is the Owner.

After all, both have green hair. I just saw "Irish people don't have green hair" and saw YKK under it for the previous WMG and...
 * There is a report of paired cross-over fics (or possibly doujinshi) called "Alphato!" and "Yotsuba no Kaidashi Kikou" about, respectively, a man who becomes the owner of a green-haired android to help socialize her in a suburban neighborhood, and an energetic orphan adopted by the proprietor of a lonely country cafÃ© and the havoc she wreaks among the neighbors.
 * Hmmm, could be! could be! In the latest chapter of Yotsubato, Yotsuba gets a small camera. In YKK, Alpha receives a camera from owner which (in both the manga and OVA) causes her to look upon it wistfully as if remembering days before days when she was a little 'bot running around Emaimachi. Also, while Alpha knew it was a camera, she didn't know how to use it until Kokone told her; Alpha wasn't up to date on the newest technologies. And furthermore, Alpha is shown to frequently carry a small pistol around everywhere. Yotsuba often carries a (water) pistol around with her wherever she goes.

Yotsuba is somehow related to the Yotsuba Group
I don't know how, just desperate to make the connection. Maybe Alpha is Misa and Rukia is...Takada.
 * And Shinji can be the guy who creates the universe Yotsuba&! takes place in as a result from Instrumentality ...uhhh...Light. Yeah, why not.
 * He wants to create a perfect world... because he subconsciously remembers what happened to the last one.
 * Maybe her real father is the founder of that group. He either named it after his daughter, whom he had to abandon somewhere for some reason, or he named her after the company before he abandoned her in hopes that maybe someday she would see the connection and seek him out.
 * So Takada is related to Reiji of The Yotsuba Group. (Look-a-like joke)

"Kiyohiko Azuma" is a collective pen name for Anonymous
She, and the entire series, was created just so that 4chan can have a mascot.
 * Dunno about the Anonymous part, given Azuma gives consistent interviews and ages appropriately in publicity photos. But it's not beyond her to have created her for that purpose, as well as for laughs.
 * Hey, just be glad I didn't throw in anything about Time Lords or A Haruhi is Yotsuba ;)
 * I'm having a hard time imagining that god!Yotsuba would invent a Yanda, or at least allow his continued existence.
 * Yanda is the devil.

The entire series is an extended exploration and Deconstruction of a possible route a girl might grow up to be the main character in a Shounen anime
Yotsuba lives with her (adoptive) father and has picked up many of his and his (male) friends' mannerisms. The reader is introduced at the turning point in her life, when she begins to get the chance to learn, from the Ayase girls, the cues and behavioral patterns that differentiate between male and female. Although the series hasn't progressed far with these developments yet, it's obvious that, in ten years, she'll be a perfect Bokukko or Tsundere for some Shounen anime series, right down to the green hair. Her lack of past is deliberate Lampshading of this true motive behind the series.
 * And among those new female models, there's even a Bokukko.
 * Or she dies just after releasing her hair from the quadri-tails in a rather weird death, becomes a Hollow, and becomes the Nel we always loved. She can't be defeated even in Hollow state because she knows Ichigo isn't evil. Just playing for laughs (or maybe tears), though.
 * So once Cerebus Syndrome kicks in, Miura will truly become Cardbo via Powered Armor and Asagi will become psychic to help her Mind Screw powers. Most of the other characters will get a similar upgrade. Except Fuka.

Yotsuba is literally The Messiah.
Yotsuba never had parents--she was granted to Mr. Koiwai by the gods to raise and teach the ways of Earth so that she'll be knowledgeable when the time comes for her to prevent the Apocalypse/lead men to Nirvana/whatever. The reason he doesn't talk about her past is to maintain The Masquerade and keep everyone believing that she's normal. Also, if people knew she was The Chosen One, they would then try to influence her and lead her down the path of evil. This also explains why she seems to know nothing about anything--not only is she young, but she is also, literally, new to planet Earth. And why she loves everything and everyone--it's in the job description.

It would explain the whole "green hair" thing.
 * So basically Yotsuba has the same plot as Princess Maker 2? Wonder when Koiwai will start sending her to magic classes..
 * Well, if Yanda is the devil... maybe she'll eventually marry him and kill him for his position? If Koiwai isn't somewhat careful, that is...

Yotsuba&! is a prequel to Azumanga Daioh
When Yotsuba grew older, she cut off two of her pigtails, dyed her hair orange, and changed her name to Chiyo. Just compare pictures of the two.
 * Or alternatively, Chiyo is her mother. Didn't Chiyo go overseas?
 * Nonsense. The "Extra Lessons" in volume 2 of the revised manga clearly show Tomo wearing Yotsuba's trademark "Kodomo Usagi" t-shirt. They both have almost the same personality and maturity level, to boot. I guess Dad isn't such a great parent, after all.
 * So Koiwai turned into an anthropomorphic yellow cat! Wait...um...
 * You'll need to explain the difference in family name and wealth.

In relation to the above guess, and with some further explanation, Torako is Sakaki's older sister.
The Sakaki parents, being well off, sent Torako to a boarding school after Sakaki was born, in preparation for her (hopefully) getting through college on her own, as did Sakaki later. Sakaki looked up to Torako's idealized image and thus became enamored with housecats (tigers' children, to a small child) and cats in general, but after Torako became less unfamiliar thanks to her interactions with Yotsuba and subsequent interactions with Sakaki, Sakaki lost the Un Favourite complex but kept the cat fixation. Yotsuba losing the nickname, dyeing her hair, and using the parental-designation-as-nickname from Mrs. Ayase for the live-in carekeepers (or extended relatives) while Koiwai is busy at the UN or working on some huge corporate translation project (perhaps heading the translation project for Azumanga Da Yo, from which Fuuka's Chiyo-chichi merchandise and Chiyo's stuffed animal originated) all add up to Sakaki making no connection between Yotsuba and Chiyo, and Sakaki never talking about her family equals Chiyo making no connection between her and Torako, as well as neither Torako nor Yotsuba and Koiwai ever giving their family names. However, Chiyo-chichi may be a result of a subconscious connection between the details Torako happened to mention about Koiwai when telling Sasaki about her friend's family and weird neighbors.
 * Sasaki?
 * Sorry, Sakaki . However, by pure coincidence and nothing to do with a certain troper's Gushing About Characters You Like, yes.
 * Spelling fixed... You can change it back if you liked it better that way.
 * I certainly agree with you on the Sakaki being Torako's sister part, I don't about the other part though. Both of 'em are Tall And Dark Bishoujo, and if you look at their eyes there's a similarity.

The police lady Yotsuba meets on her flower rampage is Tomo.
She did say that she wanted to be in the ICPO. After all, she looks the part when she dressed in the uniform for the costume race. Maybe she mellowed out some in order to get her dream job?

Yotsuba is Oasis.
She caused whatever disaster made her adoptable. In a couple of years, she'll be taken by, and they won't bother dying her hair green (or will start dying it red). Sluggy Freelance actually happens in the future, after a common way to keep people looking in their early 20s for years is discovered (as well as Owl Delivery).

Yotsuba will eventually make her own Manga about Danbo
It will happen one of 2 ways
 * She'll see a Mecha Anime at some point and will be inspired to make a manga (possibly with Ena's help) about Danbo based off of what she saw.
 * WARNING. MAY BE DEPRESSING. YOU HAVE BEEN WARNED
 * Three words to tie the sub-guesses together: Neon Genesis Evangelion.

Yotsuba becomes Nel from Bleach

 * Why else would be in her town.See?
 * Oh God, I can't unsee it now!
 * Well, if it IS him, living in wherever Yotsuba lives has done a reverse corruption on him. I wonder what would happen if she hugged him.

Yotsuba is a nature spirit, sent as an emissary to humankind

 * She's green-haired, named for a plant, hows a love for animals and plants, and is amazed by everything in human society.

'Yotsuba' isn't her real name
Okay, picture it; in a third-world country, a 2-3 year old girl loses her parents. She's too young to have formed any concrete memories of her parents, or even what her parents called her. With no friends or relatives to take her in, she wanders the streets, surviving on scraps for months.

Then along comes a young, idealistic Japanese man, eager to put his skills as a translator to work in a place where he can actually make a difference. Be it by chance or fate, the two cross paths. The man offers the girl a sweet as a token kindness, which the girl gladly devours. Upon closer inspection, the man notices the girl's incredibly disheveled appearance. Without knowing this poor girl's circumstances, and being at a complete loss for anything else to do for her, he figures the least he can do is try to wash away some of the grime off her face and hair.

The girl goes along with it, if only in the hope that the strange man will give her more food. After washing her hair the man tries to do something to fix the tangled mess it has become, but his inexpert hands somehow manage to tie off the girl's hair in four separate place. The result is four oddly-shaped lumps of hair sticking out from the girl's head. Chuckling slightly at his handiwork, the man remarks that the girl looks like a 'Yotsuba', a four leaf clover.

The girl was only partially aware of the concept of 'names'; in her experience they were something that only applied to other people. But the the man's kind smile and the warm feeling behind his words make something go 'click' inside the girl's tiny head. Thoughts of warmth, and happiness, and belonging flooded every corner of her mind. She thought of the man's gentle touch as he washed her face, and of the sweetness of the candy.

But among all these feelings, one idea dwarfed them all. An idea of such colossal importance it might as well be the meaning of life. Which, for this little girl, is exactly what this idea, this question, this hope represented.

'I'm a ... Yotsuba?'
 * tl;don't know why I read. But that makes sense. I doubt that Yotsuba is her real name. After all, she's probably not Japanese. Personally, I think her hair was just kind of like that when Koiwai first met her, and as no one knew her name, he decided to name her after her hairdo.

Yotsuba is an Oni
Her horn(s) have been damaged and haven't grown back in, so she can't fly. Yet.

Yotsuba is literally a four-leaf clover
She's suffering from a massive spiral energy overdose, due to Simon's final fight accidentally brushing a nearby galaxy. This is so obvious I see no need to further expound on it; more importantly, how long will it be before she starts warping reality through sheer cuteness?

Yotsuba is the Anthropomorphic Personification of cute.
This is Self Explanatory.

Even after she grows up, Yotsuba will still be defended by 4Chan.
I'm not sure how. Maybe she'll just look that way forever, maybe she'll grow up to be a Memetic Sex Goddess so hot that people won't be able to comprehend that she could be hotter without clothes. Because...well, 4Chan clearly has some standards.
 * So when she gets old enough to want romance, any potential suitors get scared away by by 4Chan harrasment and the poor girl never experiences romantic love?
 * It seems they've dropped the protectiveness lately. Either newfa-- I mean newcomers not knowing her as anything more than the "404 girl" or 4chan just stopped caring about anything at all anymore. Probably both.
 * I blame Four Leaf Lover.

Related to the above guess, Yotsuba will grow up to be a Hot Gym Teacher.
That way she keeps the shorts and t-shirt ensemble and the boundless energy doesn't stagnate sitting behind a desk for eight or more hours a day. She also gets to keep the Malapropers, since Azuma-san has shown that even the wisest, educated gym teachers can fail at linguistics. Her character thus takes a completely different branch from the "hotter without clothes" type, to the "would be a completely different character (who just happens to have green hair) without clothes" type.

The series will change focus when Yotsuba goes to school.
As of this writing, its mid-October in the Yotsuba&! manga, and Yotsuba will start school the following April. From that day onward, the manga will be less about Yotsuba discovering something new at home with the Ayases and more about her interaction with her teachers and fellow students. If Tomo or Osaka are any indication, putting an imaginative and energetic girl into a school setting could produce interesting results.

Yanda is a member of 4Chan.
What more needs to be said?

Miura, Shimau, and Torako are all the same person from different periods of time.
Look at the similarities. It makes perfect sense!
 * All three have a somewhat similar appearance, namely somewhat masculine female.
 * All three have similarly laid back personalities.
 * All three are friends with exactly one of the Ayase sisters.

Asagi and Torako are lesbians.
It would simply make sense, in this troper's opinion. Additionally, Yotsuba & Adolescence would seem to imply that Torako finds Fuuka attractive as well.
 * To me, it looked like she was saying Fuuka was cute the way, say, Sakaki thinks Chiyo-chan is cute (stuff it, Crackshippers), rather than how Kaorin thinks Sakaki is cute. Unless you meant throughout her presence in the chapter, in which case I didn't notice (there are other, extremely subtle clues elsewhere in the series, but nothing that couldn't be interpreted as Torako not being overtly rude to Asagi's "kid sister" [not a quote], up to and including that thing in the hot air ballon chapter). That doesn't keep Asagi and Torako from being an item, though. (...Hey, why are there No Bisexuals!?)

Yotsuba Is Death.
She's a Shinigami, or a Dead Like Me Reaper, or some sort of reaper or spirit of death who for some reason can't recall or doesn't know what she is. It would make perfect sense if she was the last reap of her reaper (after her parents, perhaps?) and he or she got their lights (I bet the reaper really wished they had soulpopped one of the parents last).

Yotsuba and Grave of the Fireflies
The universe caused Yotsuba&! to be created to restore the universal balance of happiness that was thrown off by Grave of the Fireflies.
 * Enjoy Everything?

Yotsuba can turn peoples souls into tang...
... just by hugging them. Because she is anthropomorphic personifcation of cute, as explained in an earlier WMG, and thus is so adorably heartwarming. That means that anyone, (yes anyone),who hugs Yotsuba will find that there truly is love, happiness and hope left in the world, and then there hearts will melt and become tang.
 * *SPAZZ* *SPAZZ*

Fuuka reads Azumanga Daioh
In the 5th book, Ch. 29, Yotsuba is helping collect the sheets for the Ayase's (you might have to find a physical copy for this because you can't see the back covers online for whatever reason... trust me on this one). Look carefully at what Fuuka is reading when Yotsuba gets to her room. On the front cover of the one she's holding is a humongous circular speech bubble. On the back covers of the books lying on the bed are what could pass as the outlines of Osaka (particularly these:  ).

This could explain why she has Chiyo-chichi stuff.

Kowai is Rock in the future. Yotsuba is an orphan from Roanapur, and Jumbo is an amnesiac Ginji.
One day, Rock finds orphan Yotsuba on the streets of Roanapur. He takes her in but is eventually forced to choose between giving her away to a better home or leaving the city for some place significantly safer, which he chooses to do. Yotsuba states that she's from "way left," which Thailand is in comparison to Japan. She thought Roanapur was an island as a very small child because of the water and all the boats. Rock, adopting a new identity as innocent translator Koiwai, takes her back to Japan, where he runs into Ginji, who survived his injuries but lost all of his memories except for Revy calling him "Jumbo." Out of sympathy, Rock/Koiwai creates a fictional past for them to share and finds a childless florist to pose as Jumbo's father so he doesn't go poking about trying to find his real past. A sign of his complete transformation into Jumbo is his nervousness around Asagi. Like Yukio, she's in college, and he's subconsciously afraid that if he gets close to her,. Because of this, Rock/Koiwai tries to be as vague as possible about his past so Ginji/Jumbo doesn't accidentally overhear something that will trigger his negative memories.

Torako's family name.
Many fans say it's Takino.

Koiwai is named for Koiwai Farm.
http://www.koiwai.co.jp/english/ Could this be the place in the country Yotsuba supposedly came from? (meta) Is Koiwai's name a reference? (in-universe) Is it an alias to sound more country when he's really an alien spy?

Yotsuba is the grass dog.
Not literally, but just like the grass dog in The Circus of Doctor Lao she is a hybrid of animal and vegetable, the final stage of evolution; lacking reproductive abilities because she does not need them. She's the most perfect form of life, and instinctively knowing this makes her endlessly happy. Just like the grass dog.

(Well, she does have green hair and at one point stands in a flower fridge.)

Sakaki will work at Jumbo's florist during uni.
While studying to be a veterinarian. Considering both Asagi and Sakaki are super sexy, he might try to pull some moves on her, though.

Yotsuba and Chiyo are biologically-engineered psychic supersoldier prototypes.
They were the products of an experiment in human genetic engineering, funded by the Mihama Corporation and carried out in a former Soviet republic, on an island in the Caspian Sea. Due to the language barrier between locals and Japanese, the language used at the lab was English, explaining Yotsuba's Gratuitous English and Chiyo's proficiency.

Chiyo was an early-generation prototype; after the corporation found that her only notable trait at age 1 was a prodigy-level intellect, they decided to ship her to Japan and observe her as she grew up. What they don't know is that she has the ability to unconsciously implant ideas in susceptible people (read: Osaka and Sakaki), which explains why both of them identify the strange cat-thing as Chiyo's father; she never had one, and the cat-thing is the psychic projection of her subconscious image of a father-figure.

Yotsuba was a much more sophisticated prototype, gullible and easily-controlled but with far more destructive powers; the green hair is a side effect of the process, and may indicate the presence of chloroplasts (allowing her to photosynthesize and hence to operate without food in an extended combat situation - ever wonder why she always seems so full of energy?).

Mr. Koiwai, being an investigative journalist, found out about the Mihama Corporation's lab and attempted to infiltrate it (possibly getting a job as a translator) to find out what was really going on; when an accident caused Yotsuba's powers to go out of control and destroy a large section of the lab, he rescued her and escaped.

Koiwai was later approached by agents of the corporation who offered him a bargain: if he didn't publish anything on them, they would help Yotsuba to live a normal life as his daughter. For her sake, he accepted - this is why the manga emphasizes the wonder of a "normal life" so strongly, and also why, despite doing a surprisingly small amount of actual writing, he seems to be able to support both of them fairly well.

Of course, the Mihama Corporation is still watching them very closely - what are the odds that someone close to them is in the corporation's employ?

The guy in 'Yotsuba & Cameras' really was the Nio.
That guy really was the Nio. He didn't deny it when Yotsuba asked him or even act confused at what she meant. The Nio was checking up on Yotsuba to make sure she was still being truthful. He scolded her for taking pictures without permission to which she apologized and didn't try to lie her way out of it. Even so, the Nio still roared at her and gave her a good scare. Maybe he was a little disappointed that she didn't lie after all...
 * Nio? Explain please
 * The Nio. The article even mentions the reference in Yotsuba&!.

Yotsuba is Chiyo's biological daughter and Jumbo is Chiyo-chichi in disguise
Chiyo settled down and got married in America after she completed her studies, and gave birth to Yotsuba. Shortly afterward, Chiyo (along with her husband) was killed just as Osaka predicted. Young Yotsuba, who escaped death by hiding nearby, witnessed her parent's murder and would eventually repress the memory. Since Chiyo was a somewhat wealthy Japanese citizen, and her family may have held some influence in Japan, a Japanese official of some sort was involved in the investigation, and Koiwai worked as that official's translator, which is how he met and adopted Yotsuba. Chiyo-chichi, distraught that he was unable to save his daughter, disguises himself in a giant human suit and passes himself off as Jumbo and dedicates his life to watching over his surviving granddaughter.

Yotsuba has a fungal hair infection from her life in the country to the left of Japan.
It's effectively non-transmissible, all it harms is the terminal length of Yotsuba's hair, and it would be extremely expensive and kind of traumatizing to shave Yotsuba's head and chemically treat her scalp and follicles, so Koiwai and his pediatrician and/or dermatologist decided that it was a reasonable option to leave it untreated until further development.

Yotsuba is completely bald from nuclear testing in her home country.
That's no hair, it's a symbiote!

Yotsuba has been adopted by Dr. Tenma.
Think about it...when we last left him, Tenma was working with Medecins Sans Frontieres, where he met a war orphan who would be adopted as Yotsuba. Since he came back to Japan, he changed his name legally to Koiwai to avoid attention re: the events in Monster. His 'translation' work could easily be related to telemedicine or medical transcription, and Jumbo and Yanda were fellow volunteers with MSF.

Yanda is secretly a nice guy
When Duralumin loses her voice after a dog attack and needs surgery, Yotsuba becomes near-catatonic with depression. Who gets her to cheer up but Yanda, with a series of soap bubble blowers of escalating coolness? Of course he keeps the coolest one to himself to rile her, but not when she bangs her head and won't stop crying. One begins to suspect he goes out of his way to antagonize her for her sake: Being an outlet for her frustrations, add variety and spice to her day, make her feel like the good guy, that sort of thing. Essentially Yanda is Kelso.
 * Alternatively, it's no secret. If he was really bad, Koiwai and Jumbo simply wouldn't put up with him - if you consider the interactions between them, particularly on the way to the farm/dairy, and at the yakiniku restaurant, the three of them get along rather well. Yanda's not a bad guy. Most of his antics wouldn't be out of place among friends, and indeed Koiwai and Jumbo know him well enough that they can take his behaviour in good humour, just as he intends it, even if he is a bit insensitive and occasionally takes things too far. He tries to play around with Yotsuba in the same way, but he's inexperienced with kids, and Yotsuba, being a little kid, and a somewhat odd one at that, at first takes him to be a real villain... And then, as she gets used to him, learns to play along with and enjoy his harmless antagonism - Enjoy Everything!

Yotsuba is actually six years old, not five.
Yotsuba knew how old she was (six), but Koiwai lied about her age to give her an extra year of education and cultural development before she starts school.