Personality is an NPC Trait which has a significant effect on the npc's behaviour in game, and also on which traits are likely to be picked for him, although the latter only applies if you're allowing additional traits to be added during character creation.
At heart, he's an asshole who gets a kick out of saying and doing things that are emotionally hurtful.
At heart, he's a selfish person who only really cares about himself.
At heart, he's a normal guy - his personality isn't anything out of the ordinary.
At heart, he's a true romantic.
At heart, he's a caring person who respects other people and cares about their feelings.