Laboon is an extraordinarily large whale, comparable in size to the monsters found in the Calm Belt, native to the West Blue. The Straw Hat Pirates encounter him immediately after entering the Grand Line. Fifty years ago, Laboon was left at the base of Reverse Mountain by a group of pirates who had befriended him. As his species travels in pods, he formed a strong bond with these pirates and patiently waited for their return at the entrance to the Grand Line. His caretaker is a doctor named Crocus, who usually tends to the lighthouse that guides travelers into the Grand Line.
Crocus informed the Straw Hat Pirates that Laboon's friends essentially abandoned him by escaping the Grand Line through the Calm Belt. However, this may not be entirely accurate, as Brook later described his crew as being slaughtered in the Florian Triangle. It was recently revealed that the pirates were part of Brook's crew, making him the only remaining crewmate of Laboon. Since his friends left, the old doctor Crocus has cared for Laboon. Despite knowing the truth about his friends, Laboon refuses to accept that they are gone. He has numerous scars on his head from repeatedly ramming into Reverse Mountain, believing he can break through it and return to the West Blue.
In an effort to reignite Laboon's hope, Monkey D. Luffy picks a fight with the whale by jamming the Going Merry's mast into his head. After their battle (which Luffy calls a draw), Luffy promises Laboon that he will return for a rematch. As a symbol of this promise, Luffy paints the Straw Hat Jolly Roger on Laboon's head, noting that the insignia will wash away if Laboon continues to hit Reverse Mountain. The Straw Hats leave, leaving behind two important friends.
Recently, it was revealed that the only known survivor from the pirates who had left Laboon fifty years ago is Brook, an undead skeleton whom the Straw Hats met shortly before arriving at Thriller Bark. Having reclaimed his shadow, Brook can now leave the Florian Triangle and reunite with Laboon.
(Source: Wikipedia)
