Cerberus

Cerberus is an enormous, semi-transparent, '"pure-bred three headed rottweiler" found guarding the gates to the Underworld.

History
As one of the children of Echidna, Cerberus is the only one who works for the Olympians directly. Cerberus has three heads and one body and likes to play with Mrs. O'Leary and rubber balls. One of the labors of Hercules was to bring Cerberus from the Underworld to the Earth above. Hades agreed to let Hercules take Cerberus back to the world of the living, if he could remove Cerberus from the Underworld, with only his brute strength and without killing him. In that myth, when he took him out of the living world, Eurystheus immediately told him (by messenger) that to complete the labor, he had to return Cerberus.

The Lightning Thief
Percy, Grover Underwood, and Annabeth encounter Cerberus and Annabeth manages to "tame" him by using a large red ball and some skills she learned at obedience school. Annabeth plays with Cerberus using the red rubber ball while Percy and Grover sneak past him. After Cerberus chomped the rubber ball, Annabeth tells him that they will play again someday and goes through, although she wipes a tear from her cheek because she knew that she could never come back and play with him.

Later, when speaking with Hades, Percy mentions to him that he should play with Cerberus more, saying that "he likes red rubber balls." This only enrages Hades more.

The Last Olympian
Mrs. O'Leary plays with Cerberus when Percy and Nico di Angelo are in the Underworld trying to carry out their plan for Percy to jump in the River Styx. Fortunately, he does not follow Percy and Nico to the River Styx, even though he wants Mrs. O'Leary to come back and play some more.

The Son of Neptune
When Hazel Levesque has a flash back and takes Frank Zhang with her, they see Cerberus standing in the distance while Hazel is being judged in the Judgement pavilion.

Personality
Despite being scary and fiercely dedicated to his duty, Cerberus is quite docile, playing with rubber balls with Annabeth Chase, and quickly socializing with Mrs. O'Leary. He's also very large, dwarfing even the biggest hellhound, since Percy Jackson claimed that Mrs. O'Leary (whose size is comparable that of a garbage truck) is a puppy if compared to Cerberus in terms of stature (which kind of makes sense, as Cerberus did father all Hellhounds, and being the first hellhound, they are all puppies).

Appearance
Cerberus is described as a huge black Rottweiler, twice the size of a wooly mammoth, with three heads. He wears a chain collar with three holes and has red eyes and sharp teeth and nails. Cerberus is also semi-transparent (he grows more visible the closer you are to death).