Yoake no Uta

On a certain island, there are individuals chosen to protect the common folk from creatures emerging from the sea. Elva—one of the chosen ones—fulfills this duty, beset with a cursed body that has stopped growing, and in isolation from others. When a boy named Alto learns of Elva's circumstances, he wishes to free Elva from this role. In order to support him, Alto reaches out to Elva, whom no one had dared approach before, and vows to stay by his side forever. After eight years that these two opposites had lived together like master and servant, like a family... Elva's body, supposedly only holding out for a few years, has started showing signs of recovery for an unknown reason.

(Source: Shu-Cream, translated)