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A good friend of Kouta and Kiyoka. Shigeaki had been friends with Kouta since elementary school. He is later revealed to have an interest in Kiyoka.
Late in the story, Shigeaki confided in Kouta explaining that he had fallen in love with Kiyoka and requested his opinion on what to do. Later still, he managed confessed his feelings for Kiyoka.
Shigeaki's reaction in the different endings hints at the proposed level of satisfaction of each ending.
If Kouta refused Kiyoka's demand to end his relationship with Chieri, she confided in Shigeaki and he exploded in rage. He cut his friendship with Kouta and followed through with his relationship with Kiyoka.
If Kouta accepts Kiyoka's demand and breaks up with Chieri, Shigeaki was jealous of Kouta's relationship with Kiyoka but they eventually came to good terms.
If Kouta accepts Kiyoka's demand but decides to stay with Chieri and confronts Kiyoka, Kiyoka confides in Shigeaki explaining that Kouta already had a lover but doesn't reveal Chieri's name. Shigeaki wishes the best for Kouta. If Kouta chooses Kiyoka instead, Shigeaki supports Kouta relationship with Kiyoka.
If Kouta marries Chieri, Shigeaki furthers his relationship with Kiyoka.
If Kouta requests for Mamiko's help to confront Kiyoka and says that he simply wants satisfaction, Mamiko pulls Kiyoka into his relationship with Chieri. Shigeaki revealed to Kouta that he had discovered happiness in some way.