Dr. Eli Vance
Doctor Eli Vance is a scientist who survived the Black Mesa incident. The first encounter with him was in Half-Life, when he sent Gordon for help shortly after the initial resonance cascade. (According to what he says himself, he was the scientist that assisted Gordon by using the retinal scanner, allowing him access to the Anti-Mass Spectrometer control room.) After that he took with him all that he managed to carry out of Black Mesa: His daughter Alyx, and a photo of his wife, Azian.
After the Seven Hour War, he joined (and probably led, seeming he was vital to capture for the Combine) the resistance, and was working with Dr. Judith Mossman to set up a teleporter together with Dr. Isaac Kleiner when Gordon arrived at Black Mesa East.
Eli is very important for the vortigaunts, probably for uniting the vortigaunts with the humans: he was the first human to collaborate with the vortigaunts, and has since earned their trust and is viewed as indispensable to the liberation. They praise him and believe he "almost perceives the All-in-one" (possibly their god?).
According to Dr. Breen, the Combine could conceivably have captured Eli at almost any time in the last several years, but the Combine probably recognized the importance Eli had for the unity between the vorts and the human race, and knew that a capture of him would not lead to much. Instead they used Dr. Mossman to try persuade him into the Combine way of thinking and convert him to join the Combine of his own free will instead, to demoralize and dissolve both the vortigaunt and human resistance.
He is the father of Alyx Vance, whom he can't stand to lose.
Eli is killed by an advisor at the end of Half-Life 2: Episode Two. The method in which he was killed in is believed to have given the advisor every piece of information Eli knew about the resistance, including the Borealis.