4. Cortana - Halo 4
Cortana has, alongside our steely protagonist Master Chief, been one of the mainstays of the Halo series, going from a crassly-rendered AI to a more aesthetically pleasing human-hologram hybrid, who in the fourth game is about to encounter her own demise as a bizarre form of computer dementia overcomes her. Much more so than the central narrative, Cortana's decline was easily the most engaging and affecting component of Halo 4, and even if the game attempted to greater develop Master Chief by proxy, it's Cortana who easily came off as the most interesting character. Given that she cannot be given any "treatment", it's one of the more tragic exercises in futility in recent video game lore, though 434 Industries make sure to give her an emotive send-off at the game's finale, as she briefly transforms to a hard-light hologram so that she can touch Master Chief. Her absence will be strongly felt in the next Halo game, so good luck to them coming up with a sidekick who is anywhere near this nuanced and iconic.