Born on December 20, 1970, Yukiko Koshigaya lives in her rural birthplace of Asahigaoka, Japan. Her documented age places her at 33 in 2004, 34 in 2005, and 40 in 2011. She is widowed, her late husband unnamed. Yukiko possesses pale skin, gray eyes, and black hair styled with bangs swept to the left, reaching shoulder-length. Her personality is marked by strictness, particularly directed at her middle daughter Natsumi due to Natsumi's rebelliousness, academic struggles, and impulsive nature. Yet Yukiko also displays gentleness, especially with those outside her immediate family. She interacts more harmoniously with her other children, Suguru and Komari. Yukiko maintains a well-managed household that frequently serves as a gathering spot for village children. She is mother to three children: Suguru Koshigaya, a largely silent third-year middle school student; Komari Koshigaya, a second-year middle schooler sensitive about her height; and Natsumi Koshigaya, a carefree and occasionally troublesome first-year middle schooler. She also has an implied niece referred to as "Iori". Among her friendships is neighbor Konomi Fujimiya, a fellow graduate of Asahigaoka Branch School. Yukiko herself graduated from Asahigaoka Branch School. During her student years there, she took care of Kazuho Miyauchi, who later became the village's sole schoolteacher. This mentorship mirrored the relationship Kaede Kagayama (another graduate) would later have with Kazuho's younger sister, Renge.