Chitose Hitotose, orphaned by a marine accident, lives alone in his family home. His traumatic past initially shapes a distant and guarded personality, coupled with strong fighting abilities used to defend himself against taunts about his status. Concerned by his declining grades, five of his teachers secretly move in, declaring themselves his collective "mothers." This forced cohabitation gradually softens his aloof exterior, revealing affectionate and protective instincts towards them.

Prior to this arrangement, Chitose resided in an orphanage, forming sibling-like bonds with fellow orphans Minazuki and Hazuki. Minazuki develops romantic feelings for him, which he views platonically as a younger sister. Hazuki, now a pop idol, maintains a close non-romantic relationship. These bonds highlight his complex understanding of family, further explored as the teachers integrate into his life. His development involves navigating conflicts, such as clashes with transfer student Nagatsuki Kuron—whose background mirrors his own through parental loss and combat skills—challenging his emotional resilience and identity.

Across story arcs, including the "Happy Lesson Advance" sequel and "Happy Lesson: The Final" OVAs, Chitose evolves from resistance to acceptance of his unconventional family. Key milestones include acknowledging romantic feelings for his homeroom teacher, Mutsuki, culminating in a mutual kiss that transitions their relationship from maternal to romantic. He also contends with external pressures, such as romantic interest from classmates Fumitsuki Nanakorobi and Kanna, though he remains largely oblivious to their advances. His journey concludes with a solidified bond with all five teachers and adopted sisters, emphasizing themes of found family and reconciliation with his past.