Tonari no Kyuuketsuki-san
(Novels, not manga) After taking a DNA test, the protagonist is transported to another world where humans need blood to live. She accidentally lets her guard down and gets caught up in a fight when avoiding others who are trying to eat. The blood in their mouths made her realized that vampires exist. Mochizuki Kazuo isn’t just an ordinary boy from Japan–he’s actually a vampire! It turns out he also has the same goal as Natsuki Subaru, who wants revenge against his parents’ killers, and upon meeting she tells him about her plans. He agrees to help by giving her protection, but she doesn’t know his true identity