Swapfell: Blueberry but edgy & hates his brother being lazy even more. Also it can happen by doing a few Genocide routes in Underswap
Fellswap: Red but slightly more kind hearted. He doesn't like his brother slacking off tho. Also it can happen by doing a few Pacifist routes in Underfell
There u go 2 AU's explained. But there are at least 1000 more words to it if u ask Ink or Error