Symbiotic Possession

When a demon (or any other invading, mind-controlling force) possesses someone, it can get pretty nasty. The host will have no control over their actions, they'll be forced to do the demon's bidding, or even fight their former friends. It's not a pretty picture...

...unless, that is, the demon and its host are pals.

Sometimes, a mysterious thing happens: a demon, spirit, or any other entity that is forcefully placed into someone's body (willingly or otherwise) actually befriends its host, and they form a mutual, symbiotic relationship. Maybe the demon lightens up a bit once his host tries to befriend him. Maybe he realizes that humans in general are all around good creatures that should be befriended, not taken over. Maybe he's not fond of his host, but there's a greater threat that can't be conquered unless they combine their efforts. Maybe he was never evil in the first place, and after being crammed into someone's body against his will, he felt that he might as well make friends with his fellow captive. Or hell, maybe they're both evil, and they realize that two demonic heads are better than one.

Whatever the case is, the two become less of a parasite and host, and instead two beings who view each other on equal ground and are willing to pass control of the body between each other when the situation calls for it.

Compare Willing Channeler, when the host willingly accepts the demon into his body in the first place and Heroic Host where the partnership grants fantastic ablities. Contrast with Demonic Possession, when it certainly isn't a fair relationship, and Sharing a Body, for when it's two people in the same body, and not a possessor and a host.

Anime & Manga

 * Fullmetal Alchemist has Greed, who was already shown to be a faithful friend to his chimera comrades. After he is injected into 's body, he takes it over, but 's mind survives because he willingly allows Greed into him. While Greed remains in control early on,  slowly manages to break through him. By the end of the series, the two begin to see eye to eye, throwing witty banter between each other, and Greed finally realizes that.
 * In Naruto, most of the Tailed Beasts are either treated as wild, mindless creatures, or as Chaotic Evil monsters intent on complete destruction. However, Killer B and his companion, the Eight-Tailed Ox, are on equal terms with each other, viewing each other as friends, which makes Naruto want to have the same relationship with his own Tailed Beast, the Nine-Tailed Fox.
 * It looks like he managed to do this, since his Tailed Beast is now working with him.
 * In Vampire Hunter D, D and Left Hand seem to fit the bill.
 * Yugi and Yami Yugi's relationship in Yu-Gi-Oh quickly turns into this, especially once it's discovered that Yami is less Sealed Evil in a Can and more Sealed Good in a Can.
 * Similarly, in Yu-Gi-Oh GX, Jaden and the far more demonic become this by the story's end.
 * This seems to be a recurring notion in Yu-Gi-Oh series. In Yu-Gi-Oh Ze Xal, Yuma and Astral end up sharing a body, though it's clear from the start that the latter is a Sealed Good in a Can.
 * Or at least he's suffering from severe Amnesiac Dissonance, given that the Numbers born from his missing memories are supposedly seeking to destroy the universe.

Comic Books

 * The various alien symbiotes from Spider-Man all fit this trope to some degree.
 * Venom is a complicated case. Random people bonded with the symbiote usually get completely dominated by it. Eddie Brock appears to fit this trope, but when he's separated from the symbiote his Catholic guilt kicks in and he thinks of it as a curse. Mac Gargan fit the trope, but he was bloodthirsty to begin with so the symbiote's influence didn't really matter. The current Venom,, manages to fit this trope through chemical controls and sheer will.
 * Cletus Kasady, of course, loves being bonded with the Carnage symbiote. They get along so well that they used to be considered one being.
 * And Toxin. Patrick Mulligan's relationship with his symbiote is best described as parenting.

Literature

 * In Animorphs, some Human-Controllers have this relationship. Also, most Taxxon-Controllers have this relationship with their Yeerks, because it spares them to a degree from their never-ending hunger.
 * In the last book of The Bartimaeus Trilogy, several demons manage to get themselves summoned inside several power-hungry magicians, upon which they take over their hosts and begin causing havoc. In order to save the world, allowing them to combine their abilities to combat the rogue demons.
 * Rhage and his beast slowly become this in Black Dagger Brotherhood.
 * In the Chalion books, to be a sorcerer you have to have a demon inhabiting your body. Who's in charge depends largely on whose will is stronger; the longer a demon is in the world, the stronger it is and the more likely it is to want to take control. The Bastard (who is, among other things, the god in charge of demons) has his devotees strive to remove demons from sorcerers as quickly as possible, because this is easier and less dangerous. But there are rare cases (less than one per novel) where the demon "merges" with the sorcerer's soul, and their wills become harmonious. These sorcerers are permitted to continue practicing, although the Temple likes to keep a close eye on them.
 * The Demon's Lexicon books are full of straight-up parasitic Demonic Possession which gradually kills the human host, but in.
 * The Blackened Denarii in the Dresden Files books vary between virtually full partners (Nicodemus) to hosts that are mere slaves to their demon.
 * Also, eventually come to terms with each other. Too bad it doesn't last.
 * Cormac from the Kitty Norville series is willingly possessed by the spirit of Amelia Parker, a wrongfully-executed Victorian-age wizard.
 * The Needle and Eye Of The Needle had a boy and a symbiotic alien protoplasm that were like this, until the kid got sick.
 * In the The Skinjacker Trilogy, skinjackers can temporarily possess humans. Most of the ones Allie possesses either don't know she's there because it's so brief, or, if they do realize it, they freak out and try to get her out. When skinjacking a young boy, she gets stuck in his body for a while. He initially is scared, but becomes friends with her. It's her promising to help him with his homework that wins him over.
 * Robert Heinlein's I Will Fear No Evil. The relationship between Johann/Joan and, the former owner of his new body. Later on the soul of another deceased person joins them.

Live Action TV

 * Stargate SG-1: The Goa'uld are parasitic mind controlling aliens, but a small group of them, the Tok'ra, are this. They share the body with the host and don't take hosts who are unwilling.
 * In the Star Trek universe, select Trills host a symbiote. The symbiote can live for a very long time as long as it is joined with a host. When a host dies, the symbiote is transferred to a new host. The symbiote carries the memories of the previous host with it. As a result, the new host gets the memories and knowledge from all of the symbiote's previous hosts. This results in the a fairly young host having memories of several lifetimes.

Tabletop Games

 * Averted with the Player Characters in Demon: The Fallen. The PCs may mostly be the friendly kind of demon, but they can only escape from hell and enter a mortal's body when their host is near death, either in spiritual (e.g. long-time drug addict) or physical (e.g. comatose or bleeding to death) sense. The human soul gets snuffed out or shunted elsewhere. However, the host's memories and some character traits are partially integrated into the demon's own, meaning that you can have a demon who remembers both being the celestial architect of the North Sea and having grown up as a normal human, studied journalism and raised two children.

Video Games

 * In Breath of Fire IV, Ershin and  gradually become this.
 * Occurs twice in Devil Survivor. Once with Mari, who allows Kresnik to possess her to destroy its mortal enemy Kudlak and
 * Shura from Soulcalibur IV. She's just looking for a good fight against anyone who is worth killing, the demon that possesses her is searching for the perfect host to inhabit so it can return to life. It is this unison that bolsters their power when in battle, although Shura is not the body the demon is looking for. As she is trying to satisfy her bloodlust, Shura doesn't mind this arrangement one bit.
 * Natsu (Taki's disciple) in V. She has a great demon known as Arahabaki sealed inside of her, but is apparently on good enough terms with it to call upon its power during her Critical Edge.
 * An odd play on this, and  in Tales of Graces get along just fine...because they both want to eradicate humanity. However, that is eventually revealed to be more of a case of More Than Mind Control, but  and  play it straight.
 * Wynne of Dragon Age  Anders of Dragon Age II attempts this with a Spirit of Justice, but it doesn't go well....
 * This happens in the Villainous Friendship kind of way if you choose the Demon Path in Soul Nomad and The World Eaters.
 * Skullgirls' Filia appears to regard her hair-parasite Samson in this way, judging by her in-game lines and personality. The amnesia probably helps. However, it's unclear as of yet whether Samson views Filia with the same respect.

Webcomics

 * In The Dreadful, Erin and Ith have this kind of relationship.

Web Original

 * In A Very Potter Musical, Voldemort and Quirrel evolve into this.