From Kingdom Hearts Wiki: A world of information not accessible by Gummiship
The Missile Diver is an Emblem Heartless that is found in Kingdom Hearts Final Mix. They are unique due to this Heartless forming a symbiotic relationship with Jet Balloon Heartless, which summon them to rocket at the opponent. In appearance, the Missile Diver is a blue-colored form of the Screwdiver, which holds the same style of bond with Aquatanks. While in motion, Missile Divers have a red glow at their back end, like a rocket.
A Missile Diver is a humanoid Heartless with light blue skin covered in blue-grey and blue-white zig-zag patterns. It has four fingered-hands and flippers for feet. Its head is long and torpedo-shaped, even sporting a pale green propeller on its back tip. Its forehead is decorated by angular, black spirals and it has a fin on either side of its head. Its face is pitch-black with glowing yellow eyes and its Heartless emblem is on its chest.
The Missile Diver's name references the fact that the air-bound Jet Balloon launches the Heartless like a missile into enemies. The latter part of its name references the appearance of the Heartless; it looks as if it is clad in a diving suit, complete with flippers.
While fast, these Heartless are not that difficult to beat. Simply beat the Jet Balloons that create the Missle Divers, and the rest comes easy.