Even though he should be able to die like a normal human, Kiyonosuke has been living for thousands of years, all because he accidentally "married" a merman.
But Kiyonosuke can't give up on his humanity and has been looking for a cure to immortality.
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It’s really sad and somewhat tragic in a way. It’s has mermaids who chose a chosen life mate and there’s sort of a twist in the story.
Wadatsumi no Yometori