The Turncoat is the guy who switches sides at some point to help out the other side. Can be a hero who turns bad, or a bad guy who suddenly decides to help the good guys, but usually it's just anyone who thinks that the switch will benefit them personally.
If they're discovered to have been bad the whole time, they're The Mole. If they're a bad guy who's been secretly good, they're the Reverse [...] [tvtropes.org]