First AppearanceSuper Mario RPG: Legend of the Seven Stars
The most popular character from the cult game, Super Mario RPG. For some reason, everyone loves him. There's only one problem: Nintendo doesn't have the rights to him. Technically Nintendo owns some of the rights to Geno, but Square Enix has some of the rights still.
Geno would need some imagination, but he could still have several moves based off of his moves in Super Mario RPG. Maybe his boost could make him more powerful defensively and faster for a brief time, but would require him to take a couple of seconds use it.
An obvious choice, but in order for Geno to appear, Square Enix needs to just let Nintendo use the rights. They haven't used Geno in over 15 years, so why hold on to him when they could get free publicity for him and get him popular enough to actually star in a future game.