Tomoya Mashima, sometimes spelled Tomonari Mashima, is the homeroom teacher for Class 1-A at Tokyo Metropolitan Advanced Nurturing High School. He is a tall adult male with brown hair and green eyes, typically dressed in a formal black blazer, dress shirt, black pants, and tie. His surname combines the kanji for "real, genuine" (真) and "island" (嶋), while his given name uses characters meaning "wisdom, intellect" (智) and "to be, also" (也), though its pronunciation varies between media. Years before becoming a teacher, he was a student at the same institution, belonging to the same generation as colleagues Sae Chabashira and Chie Hoshinomiya, though not in their class. After graduation, he pursued teaching like his peers, eventually forming friendships with them. Chie Hoshinomiya has referred to him as her best friend. Professionally, Mashima maintains serious dedication to his responsibilities and adheres strictly to his moral principles. He earns respect from students for his fairness and commitment to their development. He announced the rules for the Island Special Exam and later revealed the final point totals. He also explained the mechanics of the subsequent Zodiac Special Exam to students including Kiyotaka Ayanokōji, Kei Karuizawa, Teruhiko Yukimura, and Hideo Sotomura. His principled stance extends to institutional conflicts. Learning of Acting Director Tsukishiro's involvement in Chairman Sakayanagi's suspension, Mashima openly opposed Tsukishiro due to ideological differences. He was among the few faculty members willing to proactively challenge Tsukishiro's authority. During a cruise ship conversation with Chie and Sae, he expressed suspicion about Sae's interactions with Kiyotaka Ayanokōji and emphasized the importance of colleagues upholding ethical standards to avoid reporting misconduct. His actions consistently reflect a focus on student welfare, institutional ethics, and professional diligence.