Fusionfall was/is an MMO that went "let's take all your Cartoon Network faves from the 90s and early 2000s and dump them in the middle of an alien invasion", and you play as a kid that signed up to help them fight it. I tried to use the characters for other stuff when the game first shut down, but then some fans resurrected it last year and I promptly plopped them back in lol
(I also have a Wakfu OC but that's less obscure, and I haven't used her much yet...)