Aki Kitsutaka is the sister of Touru Kitsutaka and a former Burial Doll of Kurogane. She is a graduate of Rakurowa High School and is recognized as a skilled swordsman. In terms of physical appearance, she is depicted as an adolescent female with brown eyes and brown hair that reaches her shoulders.

Her role in the story is deeply tied to her tragic fate. Aki consumed all of her life force and subsequently vanished, an event that serves as a primary driving force for other characters. Her death is the main reason why her brother Touru and Tokiya aim to destroy the Second World to reach the Third World, a place where a version of Aki exists. Her legacy continues to influence events, as Kurogane Kai's two-sword skill reminds Tokiya of Aki, leading him to consider that this might be her way of trying to stop him.

During her lifetime in the First World, Aki demonstrated her protective nature and combat abilities when she saved the protagonists Tomoharu and Kanade from a stray Daughter attack at the Kitsutaka residence. In a significant gesture of support, she later gave Tomoharu her necklace when he decided to return to the Second World. This act indicates her selflessness and her desire to aid others even at a personal cost. Her notable abilities include her expertise in swordsmanship, which is highlighted as a key part of her character.