Yes, he's a vampire! In the Lore of the setting, he's a born vampire (aka never was a human, came out of the womb ready to drink blood) and in general born vampires are bigger and have more abilities. They also regenerate periodically, so he DOES age but will revert to an earlier physical state and restart aging, making him functionally immortal. Vampires also can shift into other forms, as is common in vampire lore, but the most common chosen form in our setting is a large batlike monster, which is what he usually will shapeshift into because he likes to utterly obliterate humans. The fact that his family is also one of the most physically imposing and brutal lineages is a not insignificant factor for this as well.

Nikolae CAN shift at will, but it's a painful transformation since he has to slough off his humanlike body (think The Company of Wolves and Hemlock Grove werewolf transformations - I'm sure there are other examples like that but those are the ones that immediately come to mind) and in general most of his transformations happen when he's in a heightened emotional state and can't control it. He's shifted a couple times so far in what we've written for the main story, and it is... lmao. I love him when he's Big Monster. Katya does too, although she's not ready to say that yet. 🥰