Prison Keeper

The Prison Keeper is an Emblem Heartless that is found in Kingdom Hearts II. It is one of the bosses at Halloween Town. It can swallow others to power up its attacks.

Kingdom Hearts II
A Heartless called forth in Halloween Town by Maleficent to get revenge on Sora.

It held Lock, Shock, and Barrel captive while dealing various elemental attacks.

Kingdom Hearts II
After Maleficent resurrects Oogie Boogie, she leaves Lock, Shock and Barrel behind to deal with Sora and Jack Skellington, and summons the Prison Keeper to assist them. It swallows the children to empower itself, but Sora, Donald, Goofy and Jack manage to defeat it and release the children. With their usual disloyalty, they immediately reveal Maleficent's intentions to the heroes.

Strategy
Equip all of your Aerial abilities, and maybe the Hero's Crest (unless you want something with better magic.) Also, you should equip Reflect to your shortcut menu. Fighting the Prison Keeper is all about maintaining pressure, so you should repeatedly assault it with air combos and limit attacks. If he tilts his head back approach him and use the inside combo Reaction Command to stun it.

After a while it swallows one of the three, and its attacks depend on the child:
 * Shock: The Prison Keeper will spin its arms around, rising up and then falling down while spinning around the arena, firing lasers. Its arms glow a dark yellow. This is easy to avoid, just get close and attack.
 * Lock: It will shoot large fireballs out of its mouth and hands from a distance. These fireballs can be deflected to damage the Heartless. The best way to counter this is to knock them back by using Guard or hitting them back. Reflect, although useful, is not recommended. During this attack, the Prison Keeper's arms will be red.
 * Barrel: It will use a powerful biting attack on just Sora and slash madly. Its arms glow green. Keep your distance and use Thunder.

Near the end of the fight, it will swallow all three and use a mixture of attacks: fireballs from its eyes, slash attacks, and it will spin. Don't even get near it now. Instead, use Wisdom form, keep your distance, and continue strafing (don't stop moving) until it is defeated. Donald's Limit is also helpful here. If you run out of MP, getting to the top of Spiral Hill may be the best way to start attacking the Heartless with your Keyblade.

Trivia

 * The Prison Keeper is the only boss besides the Pot Centipede to have no cutscene showing its demise.
 * The cage that it holds Lock, Shock, and Barrel in may be a reference to the cage that the trio uses in the movie, The Nightmare Before Christmas.