Seirei Gensouki: Spirit Chronicles

Amakawa Haruto, a twenty year old university student, loses his life when the bus he is on gets into an accident. As his consciousness begins to fade, his one regret is not telling his childhood friend, Miharu, how he felt. She'd disappeared after their high school entrance ceremony. Now we move to the slums of the Kingdom of Beltrum. In a corner of a filthy shack, a boy named Rio moans and wakes from a nightmare. He has lived only to take revenge upon a certain man, but now, in his mind, suddenly there are flashes of another version of himself living in a world he's never seen before—the memories of Amakawa Haruto.