Kiiko Sasaki is the sole female member of the Yamainu Squad, an Orochi extermination unit specializing in firearms. She has blond hair, typically braided over one shoulder, and light brown eyes. While adhering to the Yamainu uniform, she incorporates minor customizations for personal style.

Her personality embodies strength, dutifulness, and extreme loyalty, leading her to obey superiors' orders without question. Childhood bonds with Tenka Kumo and Sousei Abeno significantly influence her interactions, often placing her as a mediator between the two. She harbors implied romantic feelings for Tenka, feelings that deepen into profound grief following his presumed death. Despite this emotional distress, she continues her duties, though Sousei intervenes to restrict her assignments due to her instability.

Within the "Donten ni Warau Gaiden" narrative, Kiiko accompanies Tenka on his journey. Her dialogue reveals protective instincts towards him, urging him to accept support: "You’ve already done enough. Just for once, be good and let others protect you." She also acknowledges his influence over her decisions, stating, "You know you always get your way with me."

Additional background notes list her age as 24, height as 160 cm, weight as 50 kg, and blood type as A.