Mizumoto Kouya
Kouya is the future son of Arina and Yousuke. He and his twin sister came back to the past in order to prevent the end of the world, but they don't know who or what caused it. He inherited his mother's intelligence, so he tries to use his social nature in order to figure out who they need to stop. The only probelm is that he can't it keep it in his pants. Other than that, he's a positive guy who is quick to befriend others.