Shin is Sano Izumi's younger brother and a high jumper. He and Izumi were close when they were young. Izumi never contacted Shin after leaving the family. Thus, Shin felt forgotten and began to resent Izumi. He also used to be a trouble maker for a while because of Sano leaving, but they are able to get onto friendly terms once again later in the story, slightly thanks to Ashiya Mizuki. His actions toward Mizuki imply that he may have feelings for her, but he never openly confesses them.
It is implied that one reason for his hate for Sano was the added pressure put on him by their father after Sano had left home.