Dr. Isaac Kleiner is a scientist and was an employer at the Black Mesa Research Facility. He was involved with the resistance after the Combine Empire's rule and helped to bring them down. He is a good friend of Eli Vance and an uncle figure to his daughter Alyx. He is derived from the Half-Life series and appears as a minor-recurring character in City 17 Street and as a main character in the prequel Black Mesa.
Isaac Kleiner worked at the Black Mesa Research Facility in the Anomalous Materials labs overseeing the analyzing equipment. He worked with Eli Vance and was good friends with him as well as the Vance family, which included Eli's wife Azian and their daughter Alyx (who would refer to Kleiner as "Uncle").
After Black Mesa
Following the Black Mesa Incident Kleiner managed to escape with Eli and Alyx, although Azian perished.
Kleiner operated in a secret lab in City 17 working on portal technology to use against the Combine.