Vincent

Has a cold, distant personality

Is a half vampire, meaning that he was bitten but keeps his vampiric power sealed inside the gem of his earring.

He kills vampires by using traditional methods (stakes, garlic, sunlight,) and is very experienced in doing so.

Inside his vest he holds all of the weapons he needs if he comes across a vampire, even potions, poisons, and syringes. 

Very intelligent. likes to play with chemicals and sometimes performs alchemy and witchcraft

If he tastes or even smells human blood, it'll bring back his bloodthirst and make him want to kill for it. The more he gives in to his desire to hunt humans, the more his earring cracks. When iit bursts, it allows his vampire side to escape and fully take over. This would break the oath that he made to himself 200 years ago and "shame him beyond words." Luckily, he's trained himself to be neutral to the scent of human blood, but not quite the taste.

Has high morals. The main reason he kills vampires is because 200 years ago, a pack of them killed his mother. The vampires chose not to kill Vin, but to bite him so he would be forced to grow up a vampire and join them someday. He was just 13 at the time, and grew up hunting humans, but also resenting vampires and himself. Two years later, he made the oath that made his purpose and ultimate goal to either kill vampires and wipe their entire species off the planet, or find a cure and turn them all back into humans.