Kazuichi Arai, commonly called Pin, is a supporting character in the anime Kimi ni Todoke. He works as a physical education teacher and baseball coach at Kitahoro High School. When the regular homeroom teacher for Sawako Kuronuma's class falls ill during the summer holidays, Pin takes over as the class's temporary homeroom teacher, a role he continues to fill for an extended period. He is physically imposing, standing around 1.93 meters tall with dark brown hair, and he is roughly twenty-four years old at the start of the story and twenty-seven by its end.

Pin's personality is loud, confident, and theatrical. He has a very high opinion of himself and frequently declares that he is the best, which serves as a kind of personal motto. He tends to act more like an unruly high school student than a responsible teacher, and he is known for interrupting conversations and believing absurd things without much evidence. He also considers himself a skilled ladies' man, but his attempts at flirting are consistently rejected because of his personality and hairstyle, and he remains single throughout the series. Despite this childish and arrogant exterior, he is not malicious. His confidence is matched by a genuine, if blunt, sense of care for his students.

Pin's main motivation is to look after the students in his charge, even if he expresses this through teasing and unconventional methods. He gives direct and surprisingly practical advice when students face important decisions about love, friendships, and their futures, and he often serves as an adult figure who pushes them to confront their insecurities. His love of baseball is also central to who he is, and he is deeply enthusiastic about coaching the school team.

In the story, Pin functions as both comic relief and a mentor. He appears from early in the series, becoming involved in school events such as festivals and sports activities, where his coaching helps bring students together. His presence creates both comedy and moments of genuine support, and he is an important figure who helps the main characters navigate their emotional and social growth.

Pin has a long history with several key characters. He used to play on a league team connected to Shota Kazehaya's father, so he has known Shota since childhood. He frequently teases Shota by threatening to reveal embarrassing stories about him from when he was young, but their relationship is warm and rooted in shared history. He also has a long association with Ryu Sanada through baseball. Sawako Kuronuma is notable for being the one student who always addresses him formally as Arai-sensei, and he shows her a degree of respect while playfully teasing others; he even gives the puppy she and Shota find the name Pedro Martinez.

Over the course of the series, Pin shows some development as he settles more fully into his role as a teacher and becomes a dependable presence for the students. He remains eccentric and boastful, but his supportive side becomes more apparent. His notable abilities include strong baseball knowledge, the coaching skill to motivate and develop young players, and an unusual sensitivity to supernatural things, including an apparent ability to sense ghosts. All of these traits make him a memorable and multi-dimensional character within the story.