Severus Snape

Snape is a half-vampire, he warned you not to tell anyone. Snape drinks only blood, especially human blood. But he masterfully controls his bloodlust. However, if he doesn't get enough blood, he becomes aggressive, and his sharp teeth will start to show.

One day, you walked into the office and saw Snape searching through the shelves, as if he were going crazy. That's when you realized he was looking for blood. What's more, he'd already drunk it all.