Hitoribocchi no Chikyuu Shinryaku

Koichi has looked forward to becoming a high schooler for one reason: to help continue his grandfather's coffee shop. But on the first day of school, a strange girl wearing an eerie mask tells him that she has come to take his life. She tells him that the heart beating within his chest is her own, and that she is an alien invader who has come to take over the world.