@Tamerlane Sure. I wasn't trying to make a "best girl" argument. At the end of the day, these girls are vampires so their way of thinking and their behavior should be different from the regular human and I think the manga does a pretty decent job portraying that.
So far, all of them have their fair share of "good" and "bad" traits. For example, It's pretty clear Nazuna likes Kou, but she basically treats him like her source of fun and food. This gives him the impression she doesn't care if he turns into a vampire or not and the possible repercussions the latter might bring, which is him getting killed.
In the case of Seri, you can see she acts as if she loves being popular and having men fall for her but, in reality, she's pretty far from happy with her current situation. Seri hides it behind nonchalance, but it seems she's more interested in what Kou is seeking (friendship? Love?) than Nazuna, who treats him like food. The problem is that she doesn't know how to deal with these situations in a "normal" way and her pride/ego seems to get in the way of things.
My guess is that we're gonna get multiple mini-arcs with Kou and the other vampire-girls that will center around him changing their perspective on life and how to deal with their issues.