Yukiko Arashiyama, the youngest Arashiyama family member at approximately 7 years old and 110 cm tall, possesses olive/dark hair, brown eyes, and blood type O. She radiates an energetic, cheerful personality, mirroring her older sister Hotori’s enthusiasm for detective stories and mystery narratives. Yukiko frequently watches crime dramas, including reruns with known outcomes, reflecting her sustained fascination with investigative tales. She actively mimics detective work, often trailing Hotori to uncover secrets—a habit Hotori encourages despite its potential intrusiveness. Yukiko demonstrates notable persistence and cleverness during these pursuits, surprising Hotori with her resourcefulness and refusal to abandon inquiries once engaged.

As the youngest sibling, Yukiko interacts with parents Ayumu and Yukimi (whose faces are typically not shown), sister Hotori, brother Takeru, and the family dog Josephine, who appends "poko" to sentences. She occasionally collaborates with Takeru to solve minor household mysteries or orchestrates pranks that inadvertently involve neighbors. Her bond with Hotori centers on shared detective interests, though Yukiko’s curiosity sometimes disrupts Hotori’s life.

Yukiko’s investigative tendencies surface in incidents like misplacing her Nintendo DS, prompting Takeru to exploit her habit of hiding objects in frequented spots to recover it. This habit occasionally creates minor neighborhood disturbances. Her activities remain rooted in everyday childhood experiences within her family and community setting, with no significant changes documented across official series media.