Dr. Isaac Kleiner
Doctor Isaac Kleiner used to work in the Black Mesa Research Facility. Kleiner is only a character in Half-Life 2 and later. He may or may not have been the shotgun-wielding scientist from
c3a2d in Half-Life (both are seen to be about to fire shotguns at one point). Since the surrender of earth, he has been aiding the resistance through working on local-world teleportation technology in a secret lab. He also put some time aside to upgrade Gordon's HEV suit.
Dr. Kleiner keeps a de-beaked headcrab that he named after the actress and inventor Hedy Lamarr. During the teleportation incident in "A Red Letter Day", Lamarr jumped into the teleport and got teleported away to a barren wasteland, but she was later back at the lab (peeking in at Dr. Kleiner's feet as he opens the door) a week later when Gordon and Alyx returned from Nova Prospekt. As Dr. Kleiner said that there is no one like Lamarr, it is unlikely that it had been replaced by a new de-beaked headcrab, so how Dr. Kleiner and Lamarr found each other again remains a mystery to this day. In Half-Life 2: Episode Two, Lamarr is seen jumping into the rocket hatchet. It's possible that she created the 8-pound payload anomaly mentioned by Kleiner. If that's the case, she is probably dead in outer space with a gnome.