Valefor is one of the Aeons in Final Fantasy X. The names often related with evil or demon story, where he was described as "Hellish Grand Duke" and according to the stories, Valefor ruled 36 legions in the hell.
There are some theory about his appearance, some story mentioned that he shaped like a fallen angel, others mentioned Valefor had a lion body and human head. There is also a third theory which mention that Valefor had goose head and legs, body and tail of a lion hares. Curiously none of these three definitions coincide with what appears in Final Fantasy X, but it is true that his appearance in the game has an appearance of a demon.
Valefor is the protector of thieves and helps them steal. It also said that Valefor is know both of the events of the past and will happen in the future.