What about giving your character a fear? Fears are often debilitating and defining. I found a great website that lists fears, probably all the ones known to mankind.
For instance, if you have a character with ablutophobia, the fear of washing or bathing, it will really define them. First of all, they'll stink like hell. They don't buy soap. They don't have many friends, although flies love them, and they probably rarely leave their house.
If your character has botanophobia, the fear of plants, they probably live in the city, in a highrise and all their friends know, when they're shot, not to bring them flowers in the hospital. Although, they may not mind flowers on the coffin. I wonder, would they even want to be buried? It's kinda plant-like...
Here are a few of my other favorites:
- Alliumphobia- Fear of garlic. (Aren't these people called vampires?)
- Allodoxaphobia- Fear of opinions. (I know some with this problem...)
- Amnesiphobia- Fear of amnesia. (Well, would you remember anyway?)
- Anablephobia- Fear of looking up.
- Androphobia- Fear of men. (Also known as women.)
- Anglophobia- Fear of England or English culture, etc. (ha, ha, ha... I mean, that's too bad.)
- Aphenphosmphobia- Fear of being touched. (Haphephobia) (My husband says I suffer from this. I think it's because he suffers from ablutophobia. Just joking, dear.)