Luca, the Summer I Shared with You

Toono-kun’s life is thrown into confusion after a strange accident at school. A flash in the schoolyard injures several people, including his younger sister, Yayoi, who had been practicing track and field. She leaves the hospital the next day, but she no longer behaves like herself. Toono discovers that Yayoi has been replaced by Luca, a soldier from 31 years in the future. He is the only one who knows that the person now living as his sister is not really her.