Cerberus is Hades's three-headed hellhound who can be found exclusively as a boss in the Kingdom Hearts series. It appears in Kingdom Hearts, Kingdom Hearts II, Kingdom Hearts coded, Kingdom Hearts Union χ, and their remakes.
Cerberus has an angry and 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.
Cerberus is a massive, three-headed hellhound 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 fiery red eyes that slant downwards, imposing as a menace. Cerberus's legs are rather short, and each ends in a paw with three razor sharp claws. He also has a thin tail and seems to be quite muscular.
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 his 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.
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 his own cup in the Underworld, being the final opponent of the Cerberus Cup in a timed fight.
Cerberus originally appears in Greek mythology. He is depicted as a multi-headed guard dog (usually three heads) and acts as a gatekeeper of the Underworld, preventing dead souls that has crossed the river Styx from escaping. Cerberus is originally a three headed dog that Hercules had to battle in his trials in Greek Mythology and was a sibling to Chimaera and the Hydra.
In Disney's 1997 animated movie Hercules, Cerberus appears as Hades's "pet dog", and still has the same role as the gatekeeper of the Underworld. He appears when Hades is riding down the River Styx and is fed a massive piece of meat (which the three heads fight each other over). Later, Hercules tames the beast and uses him to get to Hades, inner sanctum, though this is only a temporary turn driven by fear.