Mystos, known also as The Mystery Killer, is a member of the GSFU. His true abilities are unknown, and are something of a mystery. It seems that in the place of his head, there's only a giant question mark, and two question marks for his hands.
His origin is mysterious (Much like that of Guys in general); nothing is yet known. Additionally, his powers are mysterious. It's possible he could kill someone with a simple touch, but on the other hand it's equally possible he might be unable to handle an onion. (Upcoming Game: "GSFU Game")