Alies
Barney Calhoun - The security guard from the original Half-Life. Although you saw many of his clones, in this game, there is only one Barney, and he seems to remember you ("Hey Gordon, it's me, Barney, from Black Mesa").
Dr. Isaac Kleiner - Another old friend from the original Half-Life. This time, there aren't clones, but just one Dr. Kleiner. He has a pet, Lamarr, and he seems to care more for her, than for his own life.
Alyx Vance - The daughter of Eli Vance (more about him in a few seconds). She helps you in the game at various points, and see seems to be a better shooter than most other allies. Also, many gamers have a crush on her.
Dr. Eli Vance - The father of Alyx Vance, as stated above. He has a strange leg, but he's friendly, so don't worry.
Dog - The pet of Alyx Vance. It's not a real dog, but a robot. It's extremely strong, and help you in various ways.
Dr. Judith Mossman - Don't want to spoil the original Half-Life 2 for you.
Rebels - They contain the most stupid A.I. of the game, but it's still better than nothing, right? You can take up to 4 with you, and they'll aid you by shooting enemies, and giving you health (the medics will do this, obviously).
Vortigaunts - They were enemies in the original Half-Life, but that's because they were slaves of Nihilanth. Since you saved them, by defeating Nihilanth, they are friendly now.