Hyouketsu Kyoukai no Eden

Hyouketsu Kyoukai no Eden summary:

The floating continent Orbie Clar, a land where only The Priestess's prayer can protect them from the corrupted land below, is the only place where humans are able to survive. Yumi, a barrier priestess, was waiting for a boy who would protect her; her childhood friend, Sheltis. He was expelled from the continent as a heretic, but not before promising Yumi that he'd definitely return to her side. Since then he has climbed from the corrupted garden Eden back to Orbie Clar, but will he live up to his promise and devote himself to protecting Yumi?