Winner Sinclair
About
He transfers to Karin's high school and falls in love with her at first sight, constantly making romantic overtures toward her. Despite being a vampire hunter, he does not know about Karin's true nature, as he is blinded by love and possibly also because he is not very bright.
As a child, he met a girl with whom he shared his first kiss. However, the girl he "kissed" was (unknown to him at the time) actually a younger Karin, who bit him and injected her blood into him.
There is one massive stumbling block to Winner's progress as a vampire hunter, however: he has hemophobia (aka, a fear of blood), which goes as far as feeling sick when seeing spilt red paint.
Note: Winner is an anime-only character and does not appear in the manga verison.
(Source: Wikipedia)