The hero realises that the sidekick/partner/companion's intelligence (smart vs dumb), motives ("in it for the money" vs "in it for a sense of justice"), and sensibilities (selfless vs selfish) are just too different from the hero's. Instead of completing each other in a ying/yang duality (which is what the audience expects), their differences cause too much conflict and they end up being toxic to one another.
(It may help to note that) I was inspired mainly by three things:
- Hitchcock's Psycho: (Unmarked spoiler alert, though this is a well-known twist) The film's beginning focuses on a young woman who steals some money from her boss and runs away, staying for the night in a motel. The film sets up conflict (the theft), suspense (will she get away with it?), and our protagonist (the woman). Only it turns out that she get's killed off... with over half the film's running time left to go! The film then switches to having Norman (the person who runs the motel) becoming the protagonist.
- Doctor Who: The Doctor's companions usually change throughout the series because they are either killed off, or decide that they want to go back to their families. However, there are some rare episodes where the Doctor outright "kicks out" a companion for doing something amoral, selfish, or completely insane (See: The Long Game and Planet of the Dead episodes). There are also episodes where the Doctor (at the end of an episode) offers a person he's just had a chance adventure with to come with him, but the companion refuses because they've realised adventure's not for them (See: "The Lodger").
- Dirk Gently: The novel series has Dirk travel with a different companion per book. This is because by the end of each book, the companion leaves Dirk due to not liking him or not wanting to get roped into more adventures.
Things to note:
- This character doesn't have to be likeable. In fact, I want the audience to feel happy/satisfied that this dumb character doesn't end up staying with the detective.
- This character isn't just dumb, they are also selfish, cowardly, and motivated to go on adventures for the possibility of fame and fortune.
- I'm not sure if this dumb character will be the player character, or if the game will be told in third-person.
- Steven Universe: Steven's not the brightest in his group of heroes, but he's well-meaning, kind, energetic, and wants to help people. It also helps that he's young and the three other members of his team are meant to act as parental figures to him.
- Inspector Gadget: Inspector Gadget is oblivious to danger and is often fooled by the villains in the series, having to be saved by his niece, Penny, and his dog, Brain (without even realising it). However, he still has a sense of justice, an unlimited amount of gadgets he can use, and is a loving uncle to Penny. It helps that the series (ironically) focuses more on Penny than Gadget, and that Gadget is a comedic character.
- Wallace and Gromit: See: Inspector Gadget, above.
- Goofy: Comedic character.
- Bertie Wooster (from Jeeves & Wooster): I wouldn't say he's stupid (more like an "upper-class twit"), but he's definitely not as bright as his valet, Jeeves (who possesses encyclopedic knowledge). He's also cowardly and meek, causing a lot of other characters to take advantage of him.
- Wheatley from Portal 2: An AI built for the sole purpose of making anything he's plugged into make bad decisions. A dirty coward, yet still likeable. It helps that he gives the player help, is funny, and is dependant on the player character (she has to carry him around because he has no legs).
- Lester Nyggard from 2014's Fargo: He starts out as a simple-minded, very put-upon man (like Bertie, see above), and then slowly becomes a more confident person, having some scenes show his resourcefulness as well.
- The Adventures of Paddington Bear: Not really stupid, but Paddington is quite child-like compared to how he's treated by the people around him (they treat Paddington as an adult, even referring to him as "Mr. Brown"). Since Paddington has logic similar to a child's, he makes a lot of mistakes that you wouldn't expect an adult to make (flooding a bathroom with water, getting marmalade jam on something expensive, etc.). However, he has the politeness of an adult, and never ever does something out of selfishness (unlike most children, who usually give in to their id.), and always tries to be helpful. It also helps that he's a bear who immigrated to London from "darkest Peru," and therefor isn't used to London's customs.