Ichinensei ni Nacchattara

On the day he planned to confess to his crush, Iori (nerdy, male, 2nd year high schooler) is critically injured after making a Diving Save to protect a little girl. Yume, a Lolicon scientist, secretly takes him home and rebuilds him stronger, faster as a little girl, claiming she didn't have enough "parts" to restore him to his original condition. Now he lives with her and her sane, watchful sister Mikuru (his crush) and attends elementary school with the girl he saved, keeping his true identity a secret and holding out for the day when Yume will finally turn him back. Even today, he's cutely trying his best!!

The title translates to "When I Became A First-Grader."

=== This show provides examples of: ===
 * All Men Are Perverts: Probably, since the only male characters seen are very young children, and even they find Iori attractive.
 * All Women Are Lustful: In addition to Yume, who should go without saying, every other female character is implied to have some deviant fetish or other.
 * These two tropes combine to make All People Are Lustful.
 * Alpha Bitch: Miho
 * Blind Without'Em: Yume.
 * Male Iori as shown in his character profile. One of the benefits of his new body is that 'he' has perfect eyesight once again.
 * Celebrity Paradox
 * Compulsory School Age
 * Cute Little Fangs: Iori.
 * Diving Save: And the girl Iori saves winds up becoming his "best friend" when he returns as a girl.
 * Dogged Nice Guy: Touta's crush on Iori, who can't shake him loose.
 * Gender Bender
 * Gender Bender Friendship: Iori gets to live with his crush, who thinks he's just a normal little girl. He usually turns down her invitations to bathe with her.
 * Be fair. The only time he agreed was when he was having serious identity issues.
 * Ill Boy: Rin's younger brother.
 * Lolicon: Yume. During the athletic meet chapter, Yume is joined by a mob of other candid photographers, all of whom are promptly crushed by a faction of violent teachers in what Iori remarks seems to be treated like one of the events.
 * Love Triangle: Miho --> Touta --> Iori (who wants no part in any of this)
 * Mad Scientist: Yume
 * The Mind Is a Plaything of the Body: But only once: after being surprise-kissed by Touta, Iori despairs over how fast his heart is beating in spite of the squick.
 * Naked People Are Funny: This applies about half of the time, but unfortunately some of the semi/full on nudity seems to be little more than Lolicon Fan Service.
 * Noblewoman's Laugh: Miho
 * No Good Deed Goes Unpunished: The cause of Iori's transformation and all the troubles thereafter is saving a little girl's life.
 * Nosebleed: Iori passes out from a heavy nosebleed after Mikuru walks in naked to take a bath with him.
 * The Ojou: Miho
 * One Degree of Separation: Let's see. Iori just happens to have his life saved by the sister of his oblivious crush, and is sent to the same school as the kid whom he risked his life to save. By pure coincidence, his younger sister attends that school as well. He winds up living with his crush and her sister, and after  his real family takes them in out of the goodness of their hearts, still completely oblivious to Iori's true identity. Also, a minor character that Iori and his friends encountered once lives in the same apartment complex.
 * One Steve Limit: Played With quite a bit. The people who knew Iori as a teenager and see him as a first grader don't suspect anything, because teenagers turning into children is impossible and they assume that "Iori-chan" just happens to have the same name as someone they know. Of course, both Ioris are the same person. Later,  he questions whether or not he ever was a teenager or had just received memories of someone with the same name, a la Swamp Thing. Of course, this is not the case and He Gets Better.
 * Later, there are two unrelated characters named Niwa (though their names are written differently in the original Japanese.)
 * "On the Next Episode of..." Catchphrase: Each chapter gives a brief recap of the story's premise, always ending with "Even today, he's cutely trying his best!!"
 * Plot Hole: No one ever remarks on the absence of male-Iori, not even his adoring sister, who is a regular character.
 * A Flash Back justifies his sister's obliviousness by showing that their parents had divorced when she was very young. As far as can be determined their next meeting after that was somewhere in the middle of the storyline when she would have no way of recognizing him.
 * It's also stated by Iori that he and his sister had not seen each other since the divorce, so she would not notice his disappearance in the first place (just like he didn't know she went to the same elementary school he's enrolled at during the story)
 * The Pollyanna: Rin is upbeat and cheerful for someone who lives in such a spooky house with a dark past.
 * Potty Failure: Iori, after being warned this would happen if she (a little girl) drank coffee. Subverted when it is finally discovered that he had just fallen asleep under a leaky sprinkler.
 * Princess Curls: Miho
 * Psycho Lesbian: Technically Yume. She is only ever shown to be interested in little girls, not boys, and she is more or less a Flat Character, so this is her entire personality.
 * Shout-Out - In chapter 37, Iori boasts about his talent in hiding porn magazines, and a Stand manifests behind him, complete with stats similar to how Araki ranks them in the series.
 * Single-Target Sexuality: Iori discovers he can only be aroused by Mikuru.
 * Solomon Divorce: Iori & Ion
 * Third Law of Gender Bending: mostly with underwear that is not appropriate for a first grader, as part of the Lolicon Fan Service.
 * What Do You Mean It's for Kids??: played with in the 28th chapter.
 * Woman Scorned: Iori's mom is still mad enough as it is about her former husband having cheated on her (leading to their divorce). When Iori, she's on the phone with a lawyer in seconds.