Mine would have to be The Destroyer, the leader of the antagonists.

The Destroyer is the devil king from a bad fantasy anime. In his story, the only reason he exists is to rant about the hero in the occasional post-credits sequence. His creator didn't even bother giving him a name. The way I described him to my illustrator was that the production design team cared infinitely more about this character than the writer did...and they phoned it in. What his creator did do was give him the powers of a god (probably a bad idea, under the circumstances).
So, he starts the story as a blank slate. The inciting incident of the entire book is him getting bored and booking it from his own story world. For the first half of the book, I picture most of his personality as this weird cross between Big Hero Six's Baymax and Ducktales' Webby, with his favourite move in battle is to pin people to walls with spikes of masonry.
As he finds his way in the second half of the book, he becomes considerably more terrifying...