Nanatsu no Taizai: Seven Days - Touzoku to Seishoujo

Elaine always believed humans were greedy, selfish creatures. Her brother, the King of Fairies Harlequin "King," told her as much, and this belief was only reinforced when tragedy ripped away a dear friend of theirs. This event resulted in King heading off for vengeance, leaving Elaine behind to guard the Fountain of Youth and the forest in which it resided. He promised to return, and so she waited.

Seven hundred years later, Elaine has continued to fulfill her duty. Many humans have come to seek immortality for their own selfish reasons. Each one is the same: cruel and backstabbing. But then, Elaine meets the human bandit Ban, and he defies all of her expectations; the forest protects him, and he easily gives up on the Fountain when he learns its truth. From that point on, the two of them spend seven days together, discovering that the other might be the kindred spirit they need to end their lonely days.