The Seven Deadly Sins: Seven Days Vol. 1 (manga)

Ban is a young, opportunistic thief, drawn to the Fairy King’s Forest by the rumor that it holds the most splendid, invaluable treasure of all: the Fountain of Life. But when he meets the Saint of the Fountain, a young Fairy named Elaine, he finds the treasure he most desires may not be the one he originally came to seek. Thus begins the tale of the fated romance between Ban of The Seven Deadly Sins and Elaine, the Fairy King’s sister!