Cerberus

Cerberus is is a three-headed hellhound who guards the Underworld. He first appeared as Hades's loyal pet in Disney's Hercules, as guard dog to the Underworld. Cerberus serves as a boss in Kingdom Hearts, Kingdom Hearts II, and Kingdom Hearts coded.

Kingdom Hearts
In Kingdom Hearts, Cerberus acts as a back-up plan for Hades if Cloud, whom Hades promised to help find Sephiroth for him, fails his task of eliminating Hercules and Sora.

When Sora and Cloud's battle comes to an end (regardless of who wins), Hades sees that Cloud is no longer of any use to him, and sends Cerberus in. Cerberus quickly crushes Cloud beneath its front paw when his back is turned, and is about to attack the others, but is held off by Hercules, allowing Phil, Sora, Donald, and Goofy to escape.

Cerberus has Hercules, who is carrying Cloud, trapped, but Sora comes to help, returning the favor and allowing Hercules to escape. Sora manages to defeat Cerberus, and his efforts reward him the status of "Junior Heroes". However, as Sora leaves, Hercules mentions to Phil that he had already worn Cerberus down by the time Sora intervened.

Cerberus can be fought again in full power at the Hades Cup, having four health bars instead of two (thus proving the fact that Hercules had worn it down). However, since the first time you may not have obtained Cure, it might seem easier the second time you fight it.

Kingdom Hearts II
In Kingdom Hearts II, as Sora, Donald, Goofy, and Auron are escaping from the Underworld, Hades sends Cerberus to stop them. Donald and Goofy escape as Auron fights off the creature. Sora returns to aid him, but ends up separated from Donald and Goofy, leaving him and Auron to fight the monster by themselves. The two then manage to get away, and Cerberus is trapped behind the doors as they close, but not before Sora, Donald, and Goofy taunt him by making faces. Cerberus has its own cup in the Underworld, being the final opponent of the Cerberus Cup in a timed fight.

Kingdom Hearts coded
Data-Cerberus appears as a threat to Data-Hercules, Data-Cloud, and Data-Sora. He is one of four sources of the Bug Blox corruption in the Olympus Coliseum.

Strategies
It is important to be able to climb on top of Cerberus's back, in order to avoid its Dark Breath attack and his Fireballs. Alternatively, you may use Dodge Roll to avoid Cerberus' Dark Breath attack, or even deft running. Lock on to one of the outside heads and remain to the monster's side. Use the Guard ability to deflect its fireballs. (Note: going under Cerberus' back when it uses Dark Breath will not work.) Use its back and the stairs as places to heal yourself, which is important to do often. Be sure to have a load of items like Ethers and Potions, or be sure to equip a Keyblade and items that give you at least 10 MP. Do not rely on Donald for healing, as he dies quickly, especially in the first battle. Be sure to lock on and attack the heads on the side, not the middle head. Use Dodge Roll and/or Glide (or alternatively, Superglide) often to get away from his Bite. After several attempts to bite, it will stand on two legs and roar in the air. When he does this run away from it, or be ready to jump, as it is about to use a powerful shockwave. Keep attacking the heads with your Keyblade and use a combination of Fire and Thunder magic to finish it off. While attacking the side heads, you can cast Aeroga on yourself to increase the damage. You receive the Inferno Band for winning this battle. Cerberus is by no means an easy opponent, but at the same time, it should not cause the player too much trouble. The player is only forced to face it once, but does so with Auron alone, meaning no drives. Also note that this fight is held in the Underworld, meaning drives are off limits until the Olympus Stone is obtained. The first thing Cerberus will (usually) do is jump right in front of Sora, so be ready to deliver an aerial combo. Sora will be thankful Auron is here, as his Bushido is immensely useful; although Sora should try to hit two heads with it. The Star Seeker Keyblade is recommended, as its aerial boost will be helpful while performing aerial combos. Cerberus isn't keen for a chat though, as he often swivels around leaving you nothing but his back, preventing damage. For those who use the Keyblade more then magic, just lock onto a head and start using combos, and Bushido whenever possible. If the user prefers a magical route, use thunder on one head until you have almost no MP left, then use Bushido. You are often entered into a contest of survival when Cerberus tries to eat you, when this happens, use the reaction commands to make things easier, and this contest looks brilliant as well. Cerberus has just over two bars of HP, which isn't that large, although much bigger then Sora's HP. And if this precious HP gets low, stick to the edges and use quick dash to avoid Cerberus when it jumps over to you. The Get Bonuses are an HP Boost for Auron, and Dodge Slash for Sora.
 * Kingdom Hearts
 * Kingdom Hearts II

In the Cerberus and Cerberus Paradox cups, you fight it alone, although you can still use drives. Sora can use Valor Form to hammer some sense into him, or play it safe and shoot magic bullets at it with Wisdom Form. Master Form is the most recommended form though, for some reason it seems built to fight Cerberus. All of the abilities Master Form has equipped will help Sora defeat Cerberus. Of course Final Form is the best, and Sora should use the form if he has it, but that isn't very likely.

Since the Cerberus and Cerberus Paradox cups are timed battles, you should be wary of the time constraint. If you have Final Form, use it. Every single movement is an attack in itself, and this should buy you time to keep on mashing. Do not compromise unless you have managed to take all the lesser enemies out within seconds. Cerberus in Kingdom Hearts Re:coded is quite a formidable opponent. His attacks deal good amount of damage, and he can attack Data-Sora, Hercules, and Cloud at the same time. Good timing of combos and blocking is crucial in the battle, and command decks should be filled by Cure commands so Data-Sora can quickly heal the party if the situation gets worse. Using attack commands is not recommended as they deal insignificant amount of damage to Cerberus, normal attack combo is the best way to deal damage to the three-headed dog. If several attempts on beating him fail, you can always use licenses. Regen License is particularly useful, and Auto-Life License can be given to Data-Sora to make sure the battle doesn't end soon due to Data-Sora's defeat.
 * Kingdom Hearts Re:coded

Appearance
Cerberus is a massive, three-headed dog with black fur and short, cropped ears. His heads are identical to each other, each sporting the same enormous mouths, pink tongues, and long, sharp teeth. All three heads also feature glowing red eyes. Cerberus's legs are rather short, and each ends in a paw with three grey claws. He also has a thin tail and seems to be quite muscular.

Personality
Cerberus has an angry and/or feisty personality, but will obey his master, Hades, whenever he is called into battle, as portrayed in Kingdom Hearts and Kingdom Hearts II. However, he will retaliate if Hades beats him too much.

Origin
Cerberus originally appears in Roman and Greek mythology. He is depicted as a multi-headed dog (usually 3 heads) and acts as a gatekeeper of the Underworld, preventing dead souls that has crossed the river Styx from escaping. In Disney's 1977 animated movie "Hercules", Cerberus appears as Hades's "pet dog", and still has the same role as the gatekeeper of the Underworld.

Trivia

 * Although most of the world bosses in Kingdom Hearts: Chain of Memories are the same as in Kingdom Hearts, Cerberus is replaced by Hades, who was an optional boss in Kingdom Hearts.