From Viz:
Karin is a cute little girl who also happens to be a vampire with a twist. Once a month, she experiences intense bleeding from her nose-we're talking gushers! In other words, she's a vamp with blood to spare, so rather than stealing blood from humans, she actually gives her blood to them. If done right, this can be an extremely positive experience that benefits the "victim" as much as the vamp. The problem is that Karin never seems to do things right.