Geno is a one-time character who appeared in the game "Super Mario RPG" for the SNES.
I honestly have no idea were his unwarranted popularity comes from, as he was an average character with little development. He had a couple moves that enabled him to deal 9999 damage to enemies consecutively, but that was about his only saving grace. (and personally, I never used him BECAUSE of that. Made the game far less fun)
Basically, Geno was a resident Star dweller who enhabited the body of a child's doll in order to help Mario in his quest. In my opinion, someone as completely obscure and unimportant as Geno shouldn't be used to waste a space in Brawl. However, given his overwhelming popularity, I suppose it's still possible