Kazuichi Arai, nicknamed Pin, teaches physical education and coaches baseball at Kitahoro High School, his own alma mater from roughly a decade earlier. As a high school student, he was a celebrated star baseball player, dubbed the "reckless hero of his generation," popular with peers despite offers from universities and professional scouts. He declined these opportunities to pursue teaching, driven by a deeper passion for high school baseball and a specific desire to coach at his former school. Standing 193 cm tall with a stocky build, Pin is the tallest character in the series. He possesses auburn spiky hair, tanned skin, angular eyes, and thick eyebrows. His hairstyle significantly alters his appearance; letting his hair down reveals conventional handsomeness, but his typical spiky style, combined with flamboyant clothing like bright sweaters, training pants, or jackets featuring bold embroidery, often overshadows this. He exhibits extreme self-confidence, frequently praising himself and adhering to his personal motto, "I'm the best." Initially serving as the temporary homeroom teacher for Sawako Kuronuma's class, Pin later becomes the permanent homeroom teacher after the original teacher retires. Outwardly loud, eccentric, and sometimes childish, he enjoys teasing students, particularly Shouta Kazehaya, whom he has known since childhood through Shouta's father's baseball league. Pin often leverages embarrassing stories from Shouta's past to playfully manipulate him. Beneath this seemingly superficial behavior lies a perceptive nature and genuine care for his students. He quietly observes their struggles and intervenes during critical moments with unexpected wisdom, guiding them through personal or academic challenges. For example, he dispels rumors about Sawako's "curse" and later advises her and Shouta during their romantic misunderstandings, though his methods sometimes backfire comically. His relationships extend beyond the classroom. He shares a long-standing friendship with Toru Sanada, his former baseball teammate, and frequently visits the ramen shop owned by Ryu Sanada's family, having known Ryu and Chizuru Yoshida since their childhood. During the students' third year, Pin plays a pivotal role in their career decisions, notably encouraging Ayane Yano to pursue her ambitions despite her self-doubt. His mentorship helps her recognize her potential, though he gently rejects her romantic confession near graduation, urging her to focus on her future. While flirtatious and often rejected in his own romantic pursuits due to his personality and hairstyle, Pin remains single throughout the series, adhering to strict age preferences. Pin's character subtly develops across the series. Initially perceived as self-absorbed and motivated by paychecks, his deeper dedication to teaching and student growth becomes evident. He evolves from a comical, interfering figure into a steadfast mentor who champions self-confidence and accountability, emphasizing the importance of owning one's choices. His background as a former athlete who sacrificed professional opportunities informs his advocacy for taking risks and striving toward dreams, particularly during his students' final year. This duality—combining outward bravado with quiet insight—establishes his role as an unconventional educator in the narrative.