Mahou Shoujo Ai 2

Several years after the incident, main character attends a preparatory school in Tokyo with inconvenient memories pushed out of his memory. One day he witnesses a girl fighting monsters while no ordinary human can see the girl as she is protected by the barrier. Girl tries to erase his memory, but it does not work. As a result she suggest living together as a form to keep the boy protected from monsters.

[From vndbreview]