A fair chunk of the story is never really explained. The more I think on it, the more giant plot holes I find. Very little is actually fully explained about anything. If you don't think on it for longer than a second it's not so awful, but once you think on it you begin asking a lot of questions that simply aren't answered or even really hinted at.
The anime is really pretty to look at, and when Yui's blood isn't being sucked and she's not the one talking it's actually fine to listen to.
However, Yui as a character has absolutely no agency. Which isn't unusual for this genre (visual novels adapted to anime). In visual novels the protagonist tends to have a vanilla personality so anyone can imagine themselves in that position. As a result the dialogue is intensely stilted and unbelievable. This is something that doesn't really work in my opinion in the visual novels, and definitely doesn't work in an anime. (Here is where I note that I'm a huge fan of visual novels, but that will not stop me from pointing out that particular flaw).
The other characters are okay. As per the visual novel genre they are caricatures of actual personalities, but their very limited backstories are kind of interesting and they're just good fun in my opinion.
If you're looking for a deep story that makes sense this is not for you.
If, however, you understand that you're getting into what amounts to a harlequin harem romance novel (complete with vampires and a damsel in distress) you'll probably enjoy the three hours you'll put into it.
