Asa Shigure is a central character who stands out for her vibrant energy and her role as a senior to the main protagonist. She is a student at Verbena Academy, where she is a year ahead of Rin Tsuchimi and his childhood friend Kaede Fuyou. Her background is marked by a significant secret: she is half-demon, the daughter of a human father and a demon mother, Ama Shigure. This heritage is the source of a powerful magical ability, but it also causes her severe physical frailty. As a child, her body could not handle the immense magic she inherited, leading to frequent illnesses and a deep-seated resentment toward her mother, whom she initially blamed for her suffering.

To compensate for her weak constitution, Asa developed an energetic and lively personality that often surprises those who learn of her health issues. She is known for her boisterous greetings, often greeting Rin with a cheerful "Hello!" and a hard slap on the back. Despite this somewhat tomboyish exterior, she is an exceptionally skilled cook and the head of the school's cooking club, having previously taught Kaede her culinary skills during their time at Koyou Academy. Asa is generally responsible, diligent, and seen as a reliable "big sister" figure, yet she also has a lesser-known tendency to depend on others. Her speech is often peppered with English loanwords, and she has a playful, teasing nature, especially when interacting with Rin.

Asa's primary motivation throughout the story is to live as an ordinary human being. She vehemently refuses to use her demonic magic, a decision rooted in her painful childhood and a promise to herself to reject her non-human heritage. This refusal, however, puts her life in constant danger, as the unspent magical energy within her continues to make her ill. Her role in the narrative is initially that of a supportive and fun-loving upperclassman who provides advice and encouragement to Rin and the other girls. She acts as a bridge between Rin and Kaede, helping to mend their troubled relationship. As the story progresses, her relationship with Rin deepens from a playful, love-hate friendship into a romantic one.

Key relationships define her journey. Her bond with Rin Tsuchimi evolves from teasing senior and junior to a committed partnership, where she eventually asks him to call her by name without honorifics. She shares a close friendship with the similarly cheerful Kareha, and a complex mentor-student relationship with Kaede Fuyou, who once attacked Asa out of jealousy, an incident Asa later forgives. Her relationship with her mother is central to her character; despite past anger, they share a close bond, and her mother is the one who cuts her magically-grown hair when it becomes too long.

Asa's development is triggered by her worsening illness and Rin's desperate actions. Unable to convince her to use magic to save herself, Rin takes a drastic step, using a box cutter to injure himself, declaring he cannot live in a world without her. This traumatic event finally breaks her resolve, forcing her to embrace her magical power to heal him and herself. Upon using her magic, the most noticeable change is that her hair grows extremely long, though it is later cut short again. This act represents her acceptance of her dual nature, not as a rejection of her humanity, but as a necessary step to live.

In terms of abilities, Asa is a master chef, with her cooking being one of her defining traits and a source of pride. She also possesses a vast, inherited magical power, but she deliberately suppresses it, leading to her physical weakness. When she finally does use it, it manifests as a powerful healing ability, saving Rin's life.