in the last arc, he got back to the future and discovered that he didn't marry Hiragi, but is dating Natsumi (Hiragi's sister) instead and Sana doesn't contact him anymore. He tries to avoid this future, but each time he fails. All of this happen when Sana got upset when discovers that Seiji is dating his teacher, she got heartbroken not only because she likes him, but also because Seiji lied to her. After that, Haruka broke up with Seiji because they can't mantain this teacher x student relationship. But don't worry, Seiji make up for everything and marries Hiragi at the end.