Ningyo To Ouji To Usotsuki Akuma summary:

At that moment, the merman experienced love for the first time... Meloru is a handsome merman prince with an enchanting voice that captivates everyone. His best friend is Kuraato, a scorned mendako labeled as a "demon" by his fellow sea creatures. One day, Meloru rescues the human prince Will, who has been thrown into the sea, and the two have a fateful encounter. Despite Kuraato's warnings that "befriending humans is dangerous," Meloru doesn't listen and falls deeply in love with Will...! A never-before-seen triangular love story begins between the merman prince, the human prince, and the mendako!+