Honest (3.5e Flaw)
From D&D Wiki
You cannot lie, and are disturbed when others do so.
Effect: You cannot make Bluff checks. Furthermore, any ally within sight of you takes a -5 penalty to Bluff checks. Your agitation makes it more likely others will detect falsehood.
Roleplaying Ideas: Perhaps you are compulsively honest. Or perhaps you simply believe lying to be wrong.