I'm going to say that subtle is the way to go here.

What about Geshwind Syndrome, especially manifested through hyperreligiousity? Choose a god for whom stealing is a holy sacrament, and be obsessively devout. Spend time very carefully documenting your adventures every night, while the spellcasters are meditating, and be good at paying close attention in long conversations. Get really edgy and itchy when you can't write things down at night, and get mad at people who insult your religious choices.

It's something that is a mental illness, but won't substantially interfere with adventuring, and it's not something other players are likely to have thought of?