Mimori Kiryuu is the daughter of Tadanori Kiryu, the Mainland’s top adviser for the Lost Ground, and her family is the primary financial backer of both HOLD and its Alter-user special unit, HOLY. She was born in the Lost Ground but moved away as a child, returning at the age of eighteen to take a position as a research scientist within HOLY. Her fascination with Alter powers began when she was a girl, after she witnessed a young Ryuho accidentally use his Alter during a party; instead of being frightened, she was captivated and kept a stone he created that night as a pendant, which she wears throughout the story.

Mimori is kind, idealistic, and deeply opposed to violence. She is driven by a desire to understand Alter abilities and to reunite with Ryuho, who was her childhood friend and an old flame. Despite her privileged background, she is gentle and helps nearly every major character she encounters, from diagnosing and treating Kanami Yuta’s illness to using her connections to uncover the truth about how Native Alters are treated. Her role in the story evolves from a naive researcher who believes HOLY’s mission is benevolent to a vocal critic of the organization’s brutality. After she discovers disturbing secrets about the treatment of Alter users, she is imprisoned, then rescued by Straight Cougar. Rather than returning to the Mainland, she chooses to stay in the Lost Ground. For eight months she works with Asuka Tachibana as a smuggler, bringing supplies to the Natives while searching for Ryuho. Her eventual reunion with Ryuho, who has regained a kinder disposition after losing his memories, reinforces her commitment to the Lost Ground. In the end, she severs her ties with the mainland and makes the Lost Ground her permanent home.

Mimori possesses no Alter powers herself; her notable abilities are her scientific intellect and medical knowledge, which she uses to analyze Alter phenomena and to provide care for the injured. Her primary relationship is with Ryuho, whose coldness she hopes to overcome by restoring the gentler boy she once knew. She also forms a significant bond with Kazuma, acting as a bridge between the Native Alters and the Mainland, and shares a respectful but initially tense dynamic with Scheris Adjani, who is also interested in Ryuho. Throughout the series, Mimori develops from a sheltered, optimistic researcher into a determined individual who actively challenges the system and chooses solidarity with the people of the Lost Ground.