Joyboy was introduced in the Poneglyph Robin found in the forest of the sea. After Fishman Island was saved from Hody Jones and the threat of Noah crushing the kingdom, Nico Robin asked King Neptune who Joy Boy is. The King explained that Joy Boy was a person who lived in the surface more than 800 years ago and the poneglyph was his apology for not keeping his promise to the mermaid princess. Joyboy was introduced in the Poneglyph Robin found in the forest of the sea. After Fishman Island was saved from Hody Jones and the threat of Noah crushing the kingdom, Nico Robin asked King Neptune who Joy Boy is. The King explained that Joy Boy was a person who lived in the surface more than 800 years ago and the poneglyph was his apology for not keeping his promise to the mermaid princess. Watch more here https://youtu.be/Gc9aE5vUjAs Read my blog here https://thiswriteguy.com/anime/one-piece/luffy-joyboy-greatest-war-one-piece-theory/