He first appears when Natsu and Lucy ask him for a ride to Galuna Island; he initially refuses but changes his mind upon discovering they are Mages. While at sea, he explains the curse of Galuna Island to Natsu, Lucy, Gray, and Happy, and when they arrive in the area outside the island, he suddenly "disappears".
Spoiler
He is actually the son of Moka and a resident of Galuna Island. Years ago, Bobo left the island when his father and the other villagers began to forget that they were Demons because of Moon Drip effect.
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