Sleek was the offspring of dolphins transplanted from Earth's oceans to the Santiago. Originally located in the ship's dolphinarium, like all the dolphins aboard, it, again like all aboard, was implanted at birth with a device in its melon that enabled it to concentrate the sound it made there to form drawings with bubbles. When Sky Haussmann was three years old he visited the dolphins, becoming particularly attached to Sleek, who was one of the eldest.
Eventually, as Sky's influence grew, he relocated Sleek to a secluded tank far smaller than its previous one, and in a section known only to Haussmann. As the years passed its psychotic tendencies matured, until it was described as simple a vessel of pure hate. When Sky moved the restrained saboteur to the same section he placed Sleek in charge of torturing and breaking the man, a task the dolphin enjoyed.
Finally, after he had no more use for it, Sky euthanized the creature, which it perceived as a kindness.
Its skull eventually found its way into the possession of "Red Hand" Vasquez, on Sky's Edge.