Dog

Dog (occasionally written as DØg or dØg) is Alyx's "pet" robot. Doctor Eli Vance built Dog to both celebrate and protect his daughter, Alyx, more than two decades prior to the events of Half-Life 2 when she was young. Dog started out as a relatively small, four-foot robot, but as Alyx grew up she added more and more parts to it – "upgrading" it so to speak – and it now towers eight feet high.

Characteristics
Dog, much like a real dog, is very friendly, loyal and has an intelligent personality. If the player looks at it while it's idle, it may perform a dance. Dog makes friendly and playful noises such as beeps and whines, as well as less-friendly roars when provoked. The robot's body is constructed largely of scrap metal, hydraulics, and wires, with what appears to be a scrapped Combine scanner for a head, though it has three flaps while scanners normally have four.

Dog has a zero-point energy device much like the Gravity Gun built into one hand, to pick up and throw things. Dog has amazing strength, and proves extremely useful through the course of the story, as it can throw large objects (such as vehicles) at enemies or pry open areas which are otherwise inaccessible to Gordon. Dog seems to be completely invulnerable to any damage used in-game; it never takes apparent damage from any source including weapons, fire, or explosions. However, Dog is not invulnerable; he simply has a huge amount of health to make sure he does not die in-game (as opposed to using very fast regeneration of health, as Alyx does). It is possible for him to die, but this can only be observed if the player spawns him using the console in a combat situation that he would not normally be involved in.

Dog does not have any combat AI, and all in-game scenes with Dog fighting are scripted. If a spawned Dog is approached by anything threatening, even a common headcrab, the spawned Dog will flee. If no enemies are present, it will find any nearby small objects and throw them at the player like he did in the game of catch at Black Mesa East.

Dog's character, even though important to the story, also plays a recognizable role as comic relief for the game. His battle sequences are often humorously exaggerated, where the player can see him throw large objects like cars great distances to wipe out entire squads of Combine soldiers, or furiously attack disoriented and panicking groups of enemies without caring for consequences nor taking any noticeable damage, whether it is crushing a handful of soldiers or taking a dropship head-on.

Valve commented on the Half Life 2: Episode One commentary that players love Dog, so they made him the first thing the player sees at the start of the episode.

In-game appearances
Gordon Freeman is first introduced to Dog in the Black Mesa East scrapyard. As a part of a tutorial on operation of the Gravity Gun, Alyx arranges a game of "fetch" between Gordon and Dog, using an inert Combine roller mine as the ball. The game is interrupted by a Combine assault on Black Mesa East. Dog opens the passage to an unused tunnel leading towards Ravenholm for Gordon to help him escape. While Gordon enters the tunnel, Dog stays at Black Mesa East to help Alyx and others defend the lab.

Gordon meets Dog again after he and Alyx teleport from Nova Prospekt to Doctor Isaac Kleiner's lab in the end of Entanglement. The company splits again – when Gordon and Dog set off to find and join Barney, Alyx stays with Dr. Kleiner who refuses to leave without his pet headcrab, Lamarr. As Anticitizen One begins, Dog is quick to leave Gordon behind and charge at a nearby group of Combine troops outside the lab, tossing a ruined car at them and even fighting off an APC. He then tears apart a gate for the player, allowing him to proceed. However, Dog himself goes berserk immediately after and jumps aboard a Combine dropship, departing into the unknown. It is worth noting that this whole scene – unlike other sequences where supporting characters fight alongside Gordon – is entirely scripted, and as such requires no involvement from the player at all.

Apparently, Dog eventually manages to find Barney, as Gordon meets both of them in the square toward the end of the chapter "Follow Freeman!" in a scene where Dog, in the heat of the battle, knocks over a large statue of a horse that was the player's goal during the last part of the chapter. As Barney reports to the player, Dog is probably aware of the fact that Alyx is already in the Citadel, and so tries to reunite with her, being completely uncontrollable to others. The last we see of Dog is when he, once again, opens a passage for Gordon – this time allowing him to enter the Citadel.

Half-Life 2: Episode One opens with the player under a pile of rubble from which Dog lifts him. Dog then proceeds to aid Alyx and Gordon in first contacting Eli Vance and Dr. Kleiner (by using his body as an antenna), and then resolving the issue of entering the Citadel by throwing Alyx and the player across a dividing ravine in a burnt-out van. Dog is told by Alyx to return to her father, and is not seen again for the rest of the game.

In Half-Life 2: Episode Two, dog appears late in the game and fights a Strider, with little regard for his own wellbeing. After narrowly avoiding being crushed multiple times, he manages to tear off the casing of the Strider's control mechanism and subsequently shut it down. Afterwards, he challenges the player to a race. If the player is able to beat Dog, Alyx will remark "Better luck next time, slow poke". However, if Dog wins, he will dance around triumphantly. Soon after, Dog is left outside to help patrol White Forest while Gordon and Alyx continue on to meet Eli and Kleiner.

Towards the end of the episode, Dog appears once again, but is distracted by something in the distance. During his absence, Eli, Gordon and Alyx are ambushed by Combine Advisors and Eli is shortly killed. Before the Advisors manage to repeat the horrific process on Gordon and Alyx, Dog shows up narrowly saving them. He manages to injure one Advisor considerably, whilst the other flees from his attacks relatively unscathed. As Eli dies in Alyx's arms, Dog leans over, watching in sorrow.

Reference

 * Hodgson, David. Half-Life 2: Prima Official Game Guide. Prima Games, 2004. ISBN 0-7615-4362-7.