Dr. Isaac Kleiner
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Doctor Isaac Kleiner used to work in the Black Mesa Research Facility.
Since the surrender of earth, he has been aiding the resistance through working on local-world teleportation technology in a secret lab. (Dr. Mossman gave Dr. Kleiner the sole credit for inventing this.) 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. When Gordon got teleported, Lamarr jumped into the teleport and got teleported away to a barren wasteland, but it was later back at the lab (peeking in at Dr. Kleiners feet as he opens the door) a week later when Gordon and Alyx returned from the Citadel. 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 Kleiner said that there is no one like Lamarr, it is unlikely that it had been replaced by a new de-beaked headcrab, so how Dr.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 eachother again remains a mystery to this day.
Speeches
"Well, Gordon, I see your HEV Suit still fits you like a glove. At least, the glove parts do. I've made a few modifications, but I'll just acquaint you with the essentials."