Puzzle Head

Someone that looks normal at first glance and appears to be a fully functioning human being, but in fact suffers from a severe mental illness or retardation.
Insinuates that the inside of someones head is like an unsolved puzzle and does not function correctly.

-Shit! that guy is petting that ladies dog WAY TOO HARD! whats his deal?

-Puzzle head bro, doesn't even know its a dog"