Ichirō Satō is a 17-year-old third-year student at Kawanuma West High School, sporting short brown hair and dark blue eyes. He is consistently dressed in the school uniform: a short-sleeved collared white shirt, blue tie, light grey pants, and blue sneakers. Born on May 10, 1999, Ichirō is Saki Sato's first-born child. A pivotal childhood event occurred during a Christmas trip to his mother's hometown. Believing jumping into a river would make him an adult, he plunged in and was rescued by a shy teenage girl, later identified as Kana Kojima. She took him to her house. While bathing together afterward, Ichirō discovered his first pubic hair and insisted on seeing Kojima's to confirm their shared adulthood. After her reluctant compliance, he admired her maturity and thanked her for her brave rescue. Academically, Ichirō maintains average grades, scoring a consistent "3" (on a 5-point scale) in every subject, including Japanese, Math, Science, and Physical Education. He initially pursued a classmate named Aya, but frequent accidental erotic encounters with Kojima led to Aya rejecting him. These encounters gradually redirected Ichirō's romantic interest toward Kojima. By Volume 2, he confides in his friend Rin Suzuki that he and Kojima are secretly dating, marking a key personal development. Characterized as responsible and kind, Kojima values his "kind face" and aptitude with children. Despite frequently finding himself in awkward situations with her, he consistently avoids taking advantage, demonstrating deep respect. His kindness extends to encouraging the insecure teenage Kojima, unknowingly inspiring her to pursue teaching. After graduating high school, Ichirō attends university. Kojima frequently visits him there, as shown in bonus chapters. Their relationship deepens, leading to engagement and cohabitation. A Volume 1 bonus chapter depicts Kojima insisting they switch to first-name terms, symbolizing their shift from teacher-student to romantic partners. In Volume 6, Ichirō works part-time at a store alongside Wataru Watanabe, serving as Watanabe's supervisor. His family includes his supportive mother Saki and his younger sister, Shio.