Vampire Hunter D (吸血鬼ハンターD Kyūketsuki Hantā Dī) is a series of novels written by Hideyuki Kikuchi. The series has been running since 1983, and there are now 20 books in the series. The hero (the titular D) is a dhampir (half-human, half-vampire), and a wandering hunter of the Nobility - his world's term for vampires. The books are illustrated by Yoshitaka Amano, and as of November 2007, Kikuchi and the manga artist Saiko Takaki are adapting the novels into a manga/doujinshi format.
Several elements in the Castlevania series seem to be an homage to Kikuchi's novels. Doris Lang's choice of a whip as her weapon, for example, could well have inspired the Belmonts' weapon of choice; the uncanny beauty of Alucard, akiness of D and Alucard's personalities, as well as his parentage and less-than-close relationship with his father are similar to D's situation.
In Castlevania: Curse of Darkness, a more humorous reference is made to Kikuchi's famous dhampir in one of Hector's forgeable armours. The Vampire Hunter "Comesafter C" was known to wear a Clown Suit, which is filled with the rage of those he killed.