Joy Boy

"Joy Boy" (ジョイボーイ) was first mentioned on the Ryugu Poneglyph read by Nico Robin.

History
800 years ago, the residents of Fishman Island made a promise to a great man, which later turned out to be Joy Boy. Joy Boy attempted to raise Noah with Poseidon's aid, but he broke his promise to Fishman Island. However, Fishman Island promised that they would keep Noah safe until a certain day came. On the Poneglyph is a letter of apology from Joy Boy to the mermaid princess (and Poseidon) of that time, apologizing for breaking his end of the bargain.

Fishman Island's half of the promise was almost broken during the events of the Fishman Island arc, when Luffy was asked to protect the island no matter what the cost, even if Noah had to be destroyed in the process.