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Dr. Issac "Izzy" Kleiner was a scientist at the Black Mesa Research Facility in New Mexico and a prominent leader of the Resistance against the Combine Empire on Earth. Dr. Kleiner's first appearance in the Half Life series was in a letter of recommendation for Gordon Freeman, who was Dr. Kleiner's student at MIT.

    His next major appearance is in Half Life 2, as a prominent leader of the resistance conducting teleporation technology  and giving Gordon Freeman his H.E.V suit which was lost to Gordon after the events of the original Half Life.  
    He also appears in Half Life Episode 1, warning the citizens of City 17, a city that is the main capitol of the Combine Empire on Earth, to "evacuate City 17 at once, if not sooner! I cannot state this without enough undue emphasis!" He is seen wet again in Half Life Episode 2 after the destruction of City 17, and at the Resistance base White Forest.
    To describe Dr. Kleiner is a Caucasian middle age man of average height, has large black glasses, balding black hair.  He is a brilliant, friendly and gentle man but suffers from being very absent minded at times.  He has a pet head crab named Lamar.