He is a friend from Ichitaka's class. He is first introduced in Volume 4, during a school trip to Kyoto, when he is paired with Iori. At first he appears to be a threat to Ichitaka, for he seems to be in love with Iori, but eventually, it turns out that he is a homosexual and is in love with Higemi-sensei, their teacher. Because Higemi-sensei is paired with Ichitaka, Koshinae requests for a trade, bringing the happy Ichitaka back to his crush. From that point on, Jun becomes one of Ichitaka's best friends, and an important listener when he isn't comforted with the advices he got from Teratani. Though he hates physical aggression, he is a very adept martial artist when need be.