From MangaHelpers:
Fudatsuki Kenji is known in school for two things: for being a delinquent and for having a sister complex. He constantly follows his sister Kyoko around, fixing her giant ribbon and chasing off any boys who might be interested in her. However, the truth is much stranger than people believe. He actually doesn’t have a crush on his sister-- in fact, he likes Hibino, the only girl in class who isn’t afraid of him.
The problem is that his sister is literally a monster. She’s a jiangshi (vampire), who is much stronger than an ordinary person and who needs a certain amount of blood from Kenji each day. Her ribbon keeps her in check, but if it comes off, she instinctively attacks the nearest person. Her ribbon also has a huge affect on her personality. With the ribbon, she’s irritable and sharp-tempered, but without it, she’s extremely sweet and apologetic (while she attacks). Will Kenji be able to keep his sister from hurting anyone, and maybe find some way to overcome his sister-complex reputation enough to have a chance with Hibino? [tethysdust]