Bad Seed (3.5e Flaw)
From D&D Wiki
It's hard to be good when you're tainted by evil.
Effect: You take a -6 penalty on Will saves against the spells and spell-like abilities of any creature with the Evil subtype.
Roleplaying Ideas: The most obvious application of this flaw would be to a half-fiend or tiefling meeting up with a devil or demon. Maybe the evil creature in question is slowly perverting the character to evil for their own ends and uses their fiendish heritage to manipulate them. Expect the character to either be a tortured soul trying desperately to resist their darker impulses or all too willing to go over to the dark side because it's "easier that way".