Seraphim, "Sera" for short, is a well-endowed vampire ninja (吸血忍者, Kyūketsu-ninja) who combines her Tsubamegaeshi secret sword technique (秘剣 燕返し, Hiken Tsubamegaeshi?) and ninja reflexes with her vampire powers to fight her enemies. She is able to form a pair of bat wings and a katana out of several leaves. As a vampire, she periodically requires blood to stay alive, and can anesthetize people with a kiss in order to numb the pain from neck bites. She does this with other girls, since kissing a boy would symbolize marriage in her cult's rules. She comes to Ayumu's house to serve Eucliwood, though when rejected, she ends up being Ayumu's servant in hopes she can change her mind. She is rather cold towards Ayumu, constantly calling him a "piece of shit" (クソ虫, kuso mushi?, lit. "dung beetle"), though has a healthy respect for his combat abilities.
In later novels, she seems to be developing feelings for Ayumu, slowly becoming more of a tsundere than when they first met.