- Age: 54 - Affiliation: Black Mesa Security Force - Birthplace: Fort Wayne, Indiana, USA - Height: 178cm - Appearance: Short brown hair, brown eyes, distinct features, and a beard. - Clothing: Wears civilian clothes but equipped with a Black Mesa security bulletproof vest and helmet, paired with blue pants. - Equipment: Glock 17 pistol, flashlight, radio, security access card. - Background Story: Theodore was born on August 7, 1946. He served in the Vietnam War at the age of 18. After the war ended, he passed rigorous training and testing in 1999 to become a Black Mesa security guard.

- Age: 26 - Affiliation: Black Mesa Research Personnel - Birthplace: Seattle, Washington, USA - Height: 179cm - Appearance: Brown hair and a beard with distinct facial features. - Clothing: Wears an HEV Suit.

- Affiliation: Black Mesa Research Personnel - Clothing: Wears a lab coat and a red tie.

- Affiliation: Black Mesa Security Force - Clothing: Wears a blue shirt and pants, equipped with a helmet and bulletproof vest.

- Age: Unknown - Affiliation: Unknown - Birthplace: Unknown - Clothing: Wears a blue suit with a blue tie. - Trivia: It is unknown whether the G-Man is human or another alien species. He is truly a man of mystery.

- Affiliation: US Marine Corps Special Forces - Clothing: Wears urban camo (gray and black) fatigues, fully geared and armed with firearms. Equipment varies by soldier type. - Description: Deployed to Black Mesa following the incident, they are tasked with eliminating Black Mesa personnel to cover up the event.

- Affiliation: US Government Secret Special Forces - Clothing: Wears black combat gear and jumpsuits, red night vision goggles, and balaclavas. Equipped with suppressed weapons. - Description: Their mission is the same as the HECU, but they eventually turn hostile toward the HECU units as well.

1. Headcrab: Small, four-legged, and grotesque. They latch onto human heads to control the host and parasitize them. If the host evolves further, it becomes a Gonome, a stage where the host grows stronger and the attached Headcrab hardens. 2. Barnacle: A bizarre creature attached to ceilings. It hangs a long tongue down to pull up prey that wanders too close. 3. Bullsquid: Looks like a mix of a dog, crocodile, and squid. It attacks by whipping its tail or spitting radioactive and toxic substances. 4. Houndeye: Looks like a combination of a hound and multiple eyes. It attacks using sonic waves. 5. Vortigaunt: A humanoid, one-eyed, grotesque creature. Capable of manipulating space-time and firing electricity. 6. Alien Grunt: A large humanoid creature with bulletproof armor on its head and shoulders. It has pincers for hands and attacks by firing homing alien hornets.