I didn't ready any synopsis of the story so I was surprised that it was mostly about vampires, but I really got into the story after a while and I finished the whole series pretty quickly. I don't think it had long monologues or difficult grammar, which made it easy to understand and the story was pretty engaging.
Understandable sentences but difficult words
I might be a bit biased, since I love the Anime. But the sentences overall are not casual and I think easy to understand! However I did have to loop up a lot of words and they often talk very formal which makes it easy to understand for me, but also harder, since they use grammar point I don't know. Overall it's a good manga if you're not too used to too much casual Japanese and want to learn some more words and formal grammar points.