Gomidame no Garnet

Kanae, a hitman, encounters a half vampire named Sara at a dump site and is attacked. With nothing to lose, Kanae is completely indifferent to having his blood sucked or his home invaded.

Sara, a bloodthirsty defective vampire abandoned by his parents, offers Kanae a deal: "In return for taking care of your decrepit lifestyle, become my food..." What begins as a physical relationship fueled by the arousal of bloodsucking, brings a warmth they've never known, melting away the loneliness they've both carried...

(Source: Libre, translated)