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 7 Hour War he made the stand that he'd rather let the human race face extinction than let them be slaves under the Combine, and joined (and probably led, seeming he was vital to capture for the Combine) the resistance, and was when Gordon arrived working with Dr. Judith Mossman at Black Mesa East, setting up a local teleport together with Dr. Isaac Kleiner.
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 believes he "almost percieves the All-in-one" (which seems to be their god, as the dead "return to the All-in-one" or the void).
Eli thinks highly of Freeman, and thinks Judith Mossman is a fine scientist.
According to Dr. Breen, The Combine could concievably 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. (...but that might just be what they told Dr. Mossman - the Combine might have just been after his local teleportation technology all along.)
He is the father of Alyx Vance, which he can't stand to lose.