|Family|| Ryouya Kozuki (adoptive brother)|
Soushi Asaka (adoptive brother)
Kai Nanami (spouse)
Kai Nagase (biological son)
Shin'ichirō Minato (水都 真一朗 Minato Shin'ichirō) is the school's mathematics teacher. He has an alternate personality often referred to as Minato or 'Minato-sensei', which exists in the Sukisyo games only.
Shinichirou has black spiky hair that goes slightly past his sclap, but does not touch his shoulders. He also has bright, light blue eyes. As Minato, his hair is a lot more tidy and sometimes his eyes are pitch black. Shinichirou is also shown wearing glasses though they are only shown in a few times. It's suggested that he wears them only when he is teaching.
Shinichirou is hyper, over-protective and at most times, immature. He enjoys spending time with Nanami, working out, torturing his students and spending time with the others. Shinichirou is Sora's father figure, and also the person Sora looks up to most (while he disdains Minato). Sora also comments he wants to be just like Shinichirou (as stated in the novel Tatinai Kotoba).
Shinichirou cannot keep still, and even as a child he enjoyed new adventures. He gets along with everyone around him, with exeption of his older brother Soushi and at one time his friend, Aizawa, where he comments he has a deep hatred for them.
Shinichirou was childhood friends with Kai Nanami and Aizawa. He comments that he had always loved Nanami from the first time he met him, but he was unaware of Aizawa's feeings for him, and continued spending more and more time with Nanami. The two started a romantic relation ship shortly after Nanami left Aizawa's studies. Shinichirou grew up in an orphanage with his adoptive older brothers, Soushi Asaka and Ryouya Kozuki. He only lived a few doors down from the school he attended, and was across the street from Nanami's parents clinic. Soon Ryouya left the orhanage and it was just Shinichirou and Soushi. During visitation, Ryouya would bring along presents when Shinchirou and Soushi were just children, but Shinichirou would get annoyed, wanting to play and spend time with Nanami instead.
Shinichirou had always wanted to be a detective, never one to stand still long, and always looked for new adventures. As a child, this frequently got Nanami, Aizawa and himself in to trouble with their peers. After Nanami and Aizawa left for a new school, he took on detective work and eventually got his degree in both mathematical teaching and detective officing.
Knowing he loved Nanami and with two starting a relationship romantically and physically, he wound up proposing to Nanami in the novel Tarinai Kotoba. The two are seen as married, share the same bed and even live together. Although Nanami cannot say "I love you" directly to Shinichirou, it does not bother him as he knows Nanami loves him very much.
- Shinichiro has the same name as his voice actor, Shinichiro Miki. The starting letter of their names are also the same.