Greg

"'One of these little...Buggers latched onto my buddies' head.' - Greg"

Greg was a Security Guard working a the Black Mesa Research Facility in the Administration Centre / Office Complex.

If not saved by Gordon Freeman by shooting the Security Head-crab Zombie that he mentions that a Head-crab latched onto his friends head, he will die.

After saving him (optional), he opens the gate and you can receive extra ammo and a med-kit and will accompany you through the Office Complex chapter.