Lost Souls Meet Under a Full Moon by Mizuki Tsujimura

When Hirase, a young woman from Tokyo, arranges an appointment with the Go-Between, she doesn't expect a teenager to show up. Dressed preppy-style in a duffel coat and carrying a notebook, he invites the dead back into the real world - at least, this is what Hirase has pieced together from his website page. Ushered into a luxury hotel to await her meeting, the Go-Between lays down the ground-rules: the reunion lasts one evening under a full moon; the dead person cannot be called back by anyone else, and may refuse to meet.