Koyomi Shirakawa acts as the older sister and adoptive guardian of Kotori Shirakawa. She teaches science at Kazami Academy while serving as Jun'ichi Asakura's homeroom teacher and conducting research at the Amakase Laboratory, where she oversees the Miharu android project. After concluding this project, she departs teaching to raise her child, though she occasionally visits campus to interact with former students like Jun'ichi. Koyomi maintains a close bond with Kotori, providing guidance during Kotori's emotional struggles over her feelings for Jun'ichi. She appears in the "Da Capo If" OVA, congratulating Kotori on the eve of Kotori's wedding to Jun'ichi, reinforcing her supportive role. She openly advocates for early marriage and childbearing. Physically, Koyomi sports pink shoulder-length hair, blue eyes, and glasses, often dressing in professional attire including a lab coat for work. Her family ties position her as Kotori's adoptive sister within the Shirakawa household, which adopted Kotori. She also connects to Jun'ichi through her teaching role and Kotori's eventual marriage. Her background includes belonging to a four-member family before Kotori's adoption. Post-teaching, she balances part-time work with parenting, occasionally reflecting on feeling older for first-time motherhood while remaining actively engaged in daily life.