Prosopagnosia could be fun, but an inability to recognize individuals would get rapidly very dangerous in an adventurer.

You presumably don't want something completely crippling like Korsakaff's syndrome.

A Phobia is probably the most playable mental disorder, but also predictable.

You have a couple of ways to approach this. Is your character driven by his condition, or does he strive to overcome it? Is it something that interferes with his chosen occupation, or supports it?

Examples:
Scopophobia is an anxiety disorder characterized by a morbid fear of being seen or stared at by others. Scopophobia can also be associated with a pathological fear of drawing attention to oneself. This could drive your character to become a master of stealth or a cat burglar, or you could try to overcome this fear with brash behavior. (Cue "I'm shy" from Once upon a mattress.)

Sedatephobia (fear of silence) would pretty much limit a rogues options in a lot of circumstances, whereas Sadism or Sociopathy might be a big boon in combat.