Mimori Kiryu is the daughter of Tadanori Kiryu, a wealthy investor who serves as the primary financial backer of the HOLD organization and its Alter-user special unit, HOLY. She was born in the Lost Ground but moved to the mainland at a young age, returning at the start of the story when she is eighteen years old. Mimori is a gifted scientific researcher who joins HOLY specifically to study Alter abilities, making her the only member of the organization who does not possess an Alter herself. Her fascination with Alters began in childhood when she witnessed a young Ryuho accidentally manifest his power, producing a small triangular stone that she later wore as a pendant.

Mimori is defined by her kindness, strong sense of empathy, and principled opposition to violence. She is idealistic and gentle, often putting the well-being of others above her own. Her personality sets her apart from the more ruthless members of HOLY, and she finds herself questioning the organization's methods the more she learns about the lives of the Native Alter users. Motivated by a desire to understand Alter power and a deep personal attachment to Ryuho, she returns to the Lost Ground hoping to rekindle their childhood bond. Her motivations evolve as she witnesses the harsh living conditions of the Inners and uncovers disturbing secrets about how Alter users are treated; she becomes determined to help the people of the Lost Ground and to reach the kinder Ryuho she once knew.

In the story, Mimori serves as a bridge between the mainland and the Lost Ground, offering a scientific and humanistic perspective within a conflict-driven setting. She first appears early in the series when Kazuma briefly takes her hostage to escape HOLY custody. Later, her medical knowledge proves vital when she diagnoses and treats Kanami Yuta's illness, an act that earns Kazuma's gratitude. As she grows aware of the systemic abuses in the Lost Ground, she uses her father's connections to uncover classified information, leading to her imprisonment. She is rescued by Straight Cougar and, after refusing to flee, teams up with former HOLY agent Asuka Tachibana to smuggle supplies into the Lost Ground. After Ryuho loses his memory and later recovers it, she reunites with him and witnesses his return to a more compassionate nature. Ultimately, Mimori decides to sever ties with the mainland and permanently settle in the Lost Ground as one of its Inners.

Key relationships shape her journey. Her bond with Ryuho is the emotional anchor of her character; she remembers him as a shy boy and struggles with the cold warrior he has become, but she never gives up on him. Her interactions with Kazuma begin with hostility but develop into mutual respect, especially after she helps Kanami. She shares a rivalry with Scheris Adjani, who is also romantically interested in Ryuho. Straight Cougar acts as an ally who shields her, and Tachibana becomes a partner in her secret relief efforts. Her father's wealth and influence place her in a position of privilege, but she ultimately rejects that privilege to live alongside the people she has come to care for.

Mimori has no Alter ability herself, so her notable skills lie in her scientific expertise, medical knowledge, and resourcefulness. She uses her academic training to research Alter phenomena and her intelligence to navigate dangerous situations. Her personal courage and moral convictions drive her to act against the oppressive system even without physical power. Over the course of the series, she develops from a sheltered mainland scholar into a determined and compassionate member of the Lost Ground community, embodying the theme of choosing one's own home and responsibilities.