From Kingdom Hearts Wiki: A world of information not accessible by Gummiship
The Detonator is a bomb-like Heartless. Its head is colored red, and is cracked open to show a black face with yellow eyes. Its lower body has two legs, and is completely black. On top of the head, there is a yellow fuse that is curled up, and appears to be burning at the end. The Heartless emblem is located on its head.
The Detonator has the highest defense among the other "Bomb" Heartless and counts down more frequently. The explosion it makes is also the biggest, so it is best to attack it from afar. Magic is a dynamic force when battling a Detanator; freezing a Detonator with Blizzard magic while it is counting down will freeze the countdown as well. However, using Fire magic on the Detonator will cause it to explode immediately. Therefore, Fire magic should only be used at a distance, though Blizzard magic is effective regardless of the caster's distance from the Detonator.