Tsuki wa Higashi ni Hi wa Nishi ni: Operation Sanctuary OVA

Naoki Kazumi's parents died in an accident and since he had nowhere to go, he now is living with his cousin Matsuri Shibugaki. One day up on the school's rooftop a girl falls out from the sky. The girl is called Mikoto Amagasaki, and she comes from 100 years in the future, she went back in time with hope to eventually find her missing brother. However , when Mikoto sees Naoki, she end up mistaking him for her brother.

(Source: Wikipedia)