The Maaka family matriarch. She is quite domineering and usually keeps her husband under her thumb. Her preference is the blood of liars, which she drinks like a fine wine. Calera's only known enemy in the anime is Elda Marker, as discovered in Episode 15.

(Source: Wikipedia)

Maaka Karin comes from a family of vampires living in Japan, but she is not a normal vampire. Vampires are normally supposed to drink the blood of their victims but Karin suffers from a condition that causes her body to produce excessive amounts of blood. About once a month, she has to get rid of the excess blood by giving it to her "victims." But she somehow manages to go to school like everyone else her age. 
Well, things are about to get more complicated. When a new student, Usui Kenta transfers into Karin's class, she starts producing unusually huge amounts of blood, almost as if in reaction to him. Worse, he happens upon Karin's true nature...

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 vampire. The problem is that Karin never seems to do things right!
 
(Source: Tokyopop)
 
Note: Includes 5 extra chapters.

Karin Maaka can bite a throat like a proper vampire, but where others of her kind need hot red blood, she has too much of it! Every month, she's compelled to inject blood into her victims the way a snake injects venom. And her handsome classmate Kenta Usui makes her feel like she's is going to spurt blood like a geyser. Talk about embarrassing! When Karin's latest victim turns out to be as rich as he is good looking, it causes hilarious chaos at her school. Will she ever live in peace? 
(Source: Tokyopop)

Calera Marker