A time traveller who exists as thousands of different people at once, most of whom would like the others dead.
Kang is a plural character, and that is the whole idea. Jonathan Majors plays a being who exists in variants across the multiverse, and the version stranded in the Quantum Realm was put there by the others because even they found him excessive.
The film that introduces him properly is a small one, and the choice is deliberate: he explains himself to Janet van Dyne over a kettle, and what he explains is that he has burned timelines the way other people cancel plans. His last scene is a warning about how many more of him there are.
Show detailsWhere the variants appear
The character is counted here in a single project, Ant-Man and the Wasp: Quantumania. Variants of him appear in the series Loki, where He Who Remains is one, and in that same film. The catalogue keeps variants separate from the character, which is exactly the distinction the story is built on.