Yet another aspect of many anime (particularly action / shounen types) that I enjoy is the turncoat. I couldn’t help but notice – it is used VERY frequently. I’ve seen this trope used to much that I find it very easy to identify the turncoat character(s) in anime.
What is the turncoat? It’s the antagonist character that is defeated (usually early in an anime), and returns later on in the story to join forces with the protagonist to combat a new threat. Usually they change from “enemy that must be defeated at all costs” to “friendly rival character”. The most classic example of some turncoat characters are from Dragonball Z – both Piccolo and Vegeta are villains that end up becoming core member’s of the Z fighters (the good guys).