Joy Boy

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

History
Long ago, the residents of Fishman Island made a promise to a great man, which later turned out to be Joy Boy. They promised that they would keep Noah safe until a certain day came. 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.

On the poneglyph is a letter of apology, either by or to Joy Boy, though what the letter is apologizing for is currently unknown.