Rui lost her parents in an accident when she was little, and had lived alone since then.
She can't forget the mysterious boy who comforted her on the night of her parents wake.
She started sending letters to him, the boy who she started calling Haru, even though she didn't know whether the letters would ever reach him.
After Eisuke's apartment burns, he forcibly moves to Rui's apartment, which is bad, because Eisuke is a teacher in training at Rui's school. Thus starts their life together. Rui is very reluctant at first, but soon the time together becomes very precious to Rui.
Buth then, they're discovered at school, and Eisuke leaves.