3. Hes Unique
Im not limiting this point of contention to just GTA or even gaming, but rather across all entertainment mediums. I can think of multiple mentally unstable characters, but they usually fill the role of "villain," such as with the Joker. The closest thing I can think to an insane anti-hero is Deadpool, but hes still more of a superhero. Trevor oozes originality and theres never been a character quite like him before. Hes a nasty and deplorable human being but is simultaneously loveable.