A lifeform that leaps onto human heads to infect and take over their bodies. It moves on four leg-like appendages. Its speed and health are average, but it will pursue enemies until they die. Once a host is taken over, it attacks others, growing sharp claws while the host's blood turns dark red and skin discolors. It growls and becomes more aggressive when provoked. The infection remains active unless the parasite itself is killed. A common mass-produced monster with bloodstained, scarred beige skin and short hind legs. Usually small to medium-sized.

A lifeform that infects and takes over human bodies. It moves on four tentacle-like legs. While slow, it possesses high attack power and health. Once a host is taken over, it grows sharp claws and emits low, eerie growls while attacking humans. Additional Poison Headcrabs grow on the host's back and arms, which are thrown to infect others. It is a resilient, multiplying monster with a greyish-beige surface covered in blood and grime. It has multiple hairs on its legs. Usually medium to large-sized.

A lifeform that infects and takes over human bodies. It moves on four long, slender legs. It is incredibly fast, allowing it to catch prey quickly, though it has low health. Once a host is taken over, it gains sharp claws and enhanced speed, growling every time it attacks. Like others, it must be killed at the source. It is a powerful but rarer variant. Its body is black and has a rugged, bumpy surface with long, pointed legs. Sizes vary.

Residents of the city with various personalities. Many have already been infected by Headcrabs. This group includes soldiers. Only a very few individuals retain their sense of self after infection.

Combatants tasked with eliminating the Headcrabs. They are susceptible to infection themselves. They can be found throughout the city, always carrying firearms and prepared for battle. They wear helmets, combat armor, and carry grenades.