Nickname: Pin

Height: 1.93 m

Weight: 88 kg

Blood type: B

Birthday: July 25

Sign: Leo

Life philosophy: "I'm the best"

Arai Kazuichi is the super arrogant temporary homeroom teacher for the gang's class. He is nicknamed Pin and always is called that way (except Sawako who obeys the formalities and still calls him Arai-sensei). (The previous homeroom teacher had been said to be sick after he made Sawako to do his work during the summer holidays.) He is a great fan of baseball as well as the school baseball team's coach. He used to be part of Kazehaya's father's league team, so he has known Shota since when he was young and often threatens Shota by saying he will spread all of his embarassing childhood stories. He had once said that he likes to "interrupt" people. He nicknames Kazehaya and Sawako's found-beside-the-river puppy Pedro Martinez.