Joker, Rebel Trickster

Show Me Your True Form!—Whenever a creature an opponent controls dies, you may attach
it to Joker as an Aura. It isn't a creature. Sacrifice all other Auras attached to Joker.

Joker gains +X/+X, where X is the total power and toughness of attached Auras. Joker gains the abilities of all Auras attached to him.