Eli Vance is an esteemed scientist from Black Mesa. He had been a peer of Gordon Freeman's but, after Freeman's disappearance in the first game, Freeman returned to the scene nearly 20 years younger than Eli. Thus, Gordon is now more a peer of Eli's daughter, Alyx Vance.
Eli is a major character in the Half-Life 2 series, aiding the resistance movement and working against Dr. Breen and the Combine. He can be noted for his compassion and generally soft-spoken nature, not to mention his prosthetic left leg.