Hydraball is the largest (should you consider Charonball a binary dwarf planet) and outermost known moon of Plutoball. It was dicovered in 2005 together with Nixball. It is named after the nine-headed monster from the greek mythology, as a reference to Pluto being (in the past) the ninth planet.
While not much is known about Hydraball's composition, information from the New Horizons indicate that his surface is coated in water ice. It is hypothesized that Hydraball, along with the other Plutonian moons plus Charonball, formed from the debris that came when another dwarf planet collided with Plutoball. This would mean that he should be similar in composition to Plutoball's crust.
Orbit - Rotation
Hydraball orbits the barycenter between Plutoball and Charonball in a circumbinary fashion. Like the other Plutonian moons, tidal evolution has reduced his eccentricity to nearly 0. Even then, his eccentricity is still somewhat significant, and his orbit is the most eccentric within the Pluto-Charon system. He also rotates chaotically, one of the few moons known to do so.