Merman x Human. The human can't deal with the loneliness that comes with being immortal (bc he's a merman's bride) and always try to find ways to die. However, finding out that the merman is the last of his kind holds him back because he realizes if he dies then the merman will be truly alone for eternity (merfolks only have one partner their whole life, and if that partner is a human woman, once a child is born, the merman/mermaid will die)
Wadatsumi no Yometori