Dog is Alyx Vance's "pet" robot, built by her father Eli to protect her when she was young and was eventually enhanced. He is also one of the characters from Half-Life 2 and its episodes.
Dog, much like a real dog, is very friendly, loyal, and has an intelligent and excitable personality. Similarly to Barney Calhoun, Dog provides comic relief throughout the games with his quirks and many strange and odd decisions, even dancing when he is looked at for a period of time. Dog makes friendly and playful noises such as beeps and whines, emulating the sounds made by a real dog, especially when he is around Alyx. When either he or Alyx is threatened, he can be a dangerous foe, using his immense strength and weight to smash enemies to the ground or tear them apart. This makes him a crucial asset in any battle, as he can destroy Striders with ease and hurl heavy objects at targets.
Dog's body is constructed largely of scavenged scrap metal, hydraulics and wires. He has what appears to be a scrapped Combine City Scanner for a head, though with only three flaps while normally Scanners have four. He is quadrupedal, walking on his knuckles like a gorilla.
One of his main forms of emotive outlet is his eye, which has an aperture used in much the same way as the human eye to show feelings by opening and closing. His head flaps also move according to his mood.
Alyx treats Dog much like a pet dog, calling him "boy" and using similar body language and voice as when talking to a real dog. She pats her upper thighs when calling him at Black Mesa East, and tells him to "fetch" when Gordon throws the Rollermine. Their relationship is also much like that of a dog and its master; they have a mutual trust and friendship, which has grown over the years so that Alyx now trusts Dog's opinions and hunches, as crazy as they may sometimes appear. She also has come to accept his sometimes erratic behavior, when he runs off without warning only to appear again moments later. Their relationship differs from the traditional dog and master relationship in the sense that Dog is considerably more intelligent than most real dogs, and is also much larger and more powerful than any human. However, these differences rarely come between them and their friendship seems very deep, so much so that Alyx shows many signs of affection for Dog. He also seems to have something of a sixth sense, being able to determine where Alyx is, no matter the situation. When Alyx is inside the Citadel, he is eager to get in and rescue her.
Dog has a Zero Point Energy device, much like the Gravity Gun, built into one hand so that he can pick up and throw things. He has immense strength, and proves extremely useful through the course of the story, as he can throw large objects (such as vehicles) at enemies or make way to areas which are otherwise inaccessible to Gordon. His thick plating and large size render him invulnerable to most threats he may encounter.