Yamaarashi, whose real name is Hotta, hails from Aizu and serves as the head mathematics teacher at the rural middle school where the protagonist teaches. His nickname, "Porcupine," stems from his prickly demeanor, imposing physical presence, and booming voice, which command respect and fear among students and staff. He wields a pronounced, samurai-like sense of justice, driving him to confront corruption and hypocrisy within the school.

His Aizu heritage, a region steeped in samurai loyalty and resilience, underpins his unwavering moral stance. This contrasts sharply with manipulative characters like Red Shirt, whom Yamaarashi sees as embodying elitism and superficial intellectualism. Yamaarashi's commitment to traditional values positions him as a direct foil to Red Shirt's Machiavellian tactics and relativist philosophies.

Initially perceived by the protagonist as abrasive, Yamaarashi gradually earns respect through his consistent integrity. He becomes a crucial ally when Red Shirt orchestrates the unjust transfer of the gentle English teacher, Uranari, to serve his own interests. Recognizing Red Shirt's manipulation of school policies and the principal, Yamaarashi resolves to oppose him, viewing the conflict as a battle between ethical conduct and systemic corruption.

His resistance culminates after Red Shirt engineers a false scandal to force Yamaarashi's resignation. In response, Yamaarashi collaborates with the protagonist on a plan for retribution. They ambush Red Shirt and his accomplice, Nodaiko, near the hot springs, physically confronting them as direct justice for their underhanded actions.

Following this confrontation, Yamaarashi resigns from the school and departs the rural town alongside the protagonist, returning to Tokyo. His subsequent life or career remains undisclosed. His narrative role underscores themes of moral fortitude and resistance against institutional decay, positioning him as a symbol of uncompromising ethical principles.