Carbon Dioxide & Kiss

Taiki Hisazumi is an ordinary boy who admires the aloof "goddess" of the school, Hanfu-san. One day, however, he discovers a secret she keeps hidden from everyone: she is the descendant of a vampire—specifically, a "carbon-dioxide-sucking demon" that will die unless it replenishes its supply of CO2 via mouth-to-mouth transfer! Determined to save the life of the girl he adores, Taiki vows to live as her personal carbon dioxide tank, constantly supplying her with the CO2 she needs! Neither a standard rom-com nor an erotic comedy, a "sensual comedy" begins! (Source: Kodansha, translated)