Sen-hime is a pretty young girl around Chizuru's age, with long brown hair. She is a pure-blooded oni, but because of Kimigiku, her bodyguard, Kazama Chikage doesn't chase after her as much as he does Chizuru. Sen is first introduced when she protects a child and is threatened by a group of men. Chizuru jumps in, forgetting that she cannot fight very well, and Saito Hajime steps in, knocking the men out with the blunt side of his blade, referring to a swordsmanship lesson he had with Chizuru. She later explains to Chizuru who she is and Chizuru's real heritage, even offering twice for Chizuru to come with them to be better protected.
Sen's name means wood fairy, and the suffix -hime means princess. It's possible she's called hime due to being pure-blooded, or the fact that she belongs to a high-ranking oni family.