Undying Illness

Mister Mermaid: On a full moon night, Subaru - a merman - coincidentally saved a man who sank into the ocean. After that, in order to see that man again, Subaru traded off his tail for a pair of legs and came to the surface to find Tatsuki - the man's name. Despite of his throbbing feet and his lost voice, Subaru was happy to be able to stay by Tatsuki's side. However, his happiness soon faded away when he found out the reason why Tatsuki was at the bottom of the ocean that day.

Fuji no Yamai (Undying Illness): Byakuya had an incurable undying illness, which means he couldn't die. Gin, his best friend, made a promise that he would take care of Byakuya until death and the promise had been kept for 16 generations. However, Gin the Sixteenth had no intention of making babies for the next generation; instead, he kept his promise with Byakuya in a way that none of the previous Gins had done.

NOTE: A sequel to Mister Mermaid, "Mister Mermaid After That," was published in doujinshi.