(LOVED trope) 'offscreen redemption arcs', a.k.a when a character last seen as a villain comes back after a timeskip and is now a supporting good guy.

I like this trope because it affirms that everyone has the ability to grow into a better person, even in the background when the story is not focusing on them. It also gives the audience room to headcanon exactly what happened to them during the timeskip.