Party piñatas are traditionally shaped as donkeys and are candy filled with a hard exterior. Someone who acts like an ass but is actually a sweet person can be described as a piñata.
You: "Ugh, Sean is such a jerk
! I don't know how you're friends with him."
Me: "Actually, he's a piñata. He puts on this front as a womanizer
and a douche bag
, but is the nicest person."