Yuuta

500 year old immortal. He ate the flesh of a mermaid out of curiosity when he and his fellow fishermen had captured one, but out of all the fishermen who ate a piece, only Yuta had survived. His comrades had either been poisoned or turned into soul-less monsters. Yuta, being scared that he will meet the same fate, goes into denial before simply forgetting about ever eating the mermaid's flesh. He takes a wife and lives normally until he discovers that while she grew older as the years go by, he hasn't aged since the day he ate the flesh. He hears that a mermaid will be able to help him turn normal again, thus, travels endlessly in search of a way to become mortal. Along his travels, he meets a girl named Mana, another immortal who was fed mermaid flesh by the ones who raised her. He saves Mana from the mermaids who were planning to devour her and takes her in as a traveling companion.