Under The Same Roof With My Ex-Girlfriend

Kurayami is your average, run-of-the-mill high school student who has a very limited friend circle and spends most of his time reading books. His life had been going fine and all but when his relatives die and the only girl the now-dead couple had would be living with his family from now on. It would have been all right, if not for the fact that the girl was his age! And the cherry on the cake - she is his ex-girlfriend !?