Leif and Jessica were two vampires that appeared in both the anime and manga (along in the OVA series).
Leif (ハガー) was the male vampire, with pale skin and black hair.
Jessica (ジェシカ) was the female vampire of the duo, hair dyed a blue color. She too had fair skin.
They are newly turned and quite weak for vampires. They first appeared in the first episode of the OVA making out after murdering several people. Leif was killed by Alucard and Jessica was terminated by Seras Victoria. They are cruel, trigger happy, sadistic and psychotic as they ruthlessly killed innocent people without reason. They are also shown to be sexually deviant, indecent and very vulgar in the television series, as shown after their recent killing spree Jessica is shown off-screen giving Leif fellatio.
Leif is shown to be the most ruthless and psychotic as he seems to be the one who calls the shots. His girlfriend, Jessica, only seems to follow him without a second thought. In the anime they are called "Bonnie and Clyde".