Basically I recommend the manga here 1000% The story of Karin is actually quite moving and quirky from beginning to end. I actually dislike the anime because it is so much filler and the characters don't really grow as much.

In the manga you actually learn about Kenta Usui and his mother. You see the struggles and the romance that builds between him and Karin. You actually learn some history of the Maaka Vampires and even meet other families. You learn about Karin's condition of releasing blood and what it means to the vampires as well as to her future among them. It's a much more solid story and leads you through the roller coaster of the developing relationship and eventual sacrifices that must be made. I loved the whole of it.

The anime was weak in comparison stripping a lot of the dramatic and familial tensions for cheap filler and stripping some characters and plots out entirely. I feel that the characters don't really grow at all there.

So if you engage with this wonderful vampire story please just read the manga. You'll be glad you did.