Maaka Karin
Maaka Karin is a 16-year-old "blood-producing" vampire whose body produces too much blood. As such, she does not suck blood from her "victims"; rather, she injects her surplus blood into them. This has the effect of making her victims very lively and positive. To make matters worse, her blood levels increase dramatically whenever she senses misfortune in those around her, or if she gets embarrassed about something. This results in frequent and excessively overwhelming nosebleeds, followed by anemic symptoms such as being lightheaded, which cause frequent visits to the school nurse's office so she can rest.
Usui Kenta
Usui Kenta is a student that transferred to Karin's high school. He is also her workmate. Kenta is the only human that knows of Karin's vampiric nature. He learned what she was after witnessing her biting three victims, one of which was his own mother. He has promised to keep this knowledge a secret. As his unhappiness causes Karin's blood to rapidly increase, she has taken it upon herself to cheer him up by making him a bento every day. He could not afford to buy his own lunch and is therefore grateful to her.