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Dog is a character that first appears in the Valve-developed Half-Life 2 game and is also one of the main protagonists in the subsequent 2 episodes that follow on from that original game ( Half Life 2: Episode 1 and Half-Life 2: Episode 2). The player first encounters Dog upon reaching the base, which is
home to Eli Vance and his daughter Alyx Vance. It is explained that Dog was built by Eli Vance in order to protect his daughter and originally started life as a far smaller and less intelligent version, but which has grown in size and strength as Alyx has also grown up so that Dog is able to continue to protect Alyx as she fulfills a more active role in the resistance.
Dog clearly has an advanced artificial intelligence, giving it precision control of its movement and interactions with its environment, but importantly allowing it to assess threats to itself or others and then finding a way to counter them. It also has the ability to recognize specific persons such as Alyx and Gordon visually and also audibly, since it can respond to and obey Alyx's commands. Dog also appears to have a personality that goes beyond traditional AI because of the way in which it appears to be able to have 'fun' (such as when Gordon plays 'catch' with Dog when first meeting him) and express affection and loyalty in its relationship with Alyx despite its inability to communicate verbally.
Dog has great strength, speed and agility which means it is able to take down Combine drop ships and Striders on its own, often while saving Gordon Freeman and Alyx in the process. It seems that Dog now treats Gordon as a friend and protects him in the same way that it protects Alyx.
Fun fact: Players are introduced to Dog at the same time as the gravity gun. They learn to use the gravity gun by playing catch with Dog.