She's a Chuunibyou, and I Was the Only One Who Believed Her, Then the Bathroom Exploded

Lin Sha Xie is an ordinary high school boy. Then a dark-skinned female transfer student who excels at sports suddenly claims to know him. His deskmate also happened to transfer away the day before, making him wonder if her claim that she came from the future is true. She even confesses to Lin Sha Xie right from the start, but he doesn't take it seriously. He only starts to wonder after getting stuck standing as punishment for going to the bathroom. Maybe she isn't from the future, but she might really not be human.