Laboon is an extremely large whale (of comparable or greater size than the monsters in Calm Belt), native to West Blue, whom the Straw Hat Pirates encounter immediately after entering Grand Line. Fifty years earlier, Laboon had been left at the bottom of Reverse Mountain by a group of pirates who had befriended him. Since his species travels in pods, he had forged a powerful bond of friendship with these pirates, and he patiently waited for them at Grand Line's entrance. His caretaker is a doctor named Crocus, who normally mans the Lighthouse that introduces travelers into Grand Line.

Crocus revealed to the Straw Hat Pirates that Laboon's friends effectively abandoned him by fleeing the Grand Line through the Calm Belt. (although this may not be true as Brook described his crew as being slaughtered in the Florian Triangle) The pirates were recently revealed to be the crew of Brook the skeleton, leaving him as Laboon's only crewmate left. The old doctor has cared for Laboon ever since his friends left him. Although Laboon knows the truth about his friends, he refuses to admit to himself that they are gone. Laboon has many scars on his head from slamming into Reverse Mountain, for he believes he can destroy the mountain and return to West Blue.

Monkey D. Luffy, in an effort to give Laboon a renewed sense of hope, picks a fight with the whale by jamming the Going Merry's mast into his head. After the battle (which Luffy calls a draw), Luffy promises to Laboon that he will return to see him, under the guise of wanting a rematch. As a sign of this, Luffy paints the Strawhat Jolly Roger on Laboon's head, commenting that the insignia will wash away if Laboon hits Reverse Mountain further. The Straw hats depart, leaving behind two important friends.

(Source: Wikipedia)