You, Kleiner and Eli Vance (alongside his daughter) escaped Black Mesa after a "resonance cascade" has arrived at the facility. Then he was then lonely experimenting his portal technology (and theories), when he's actually was told about Gordon trying to stop the Combines. He then notices a super-portal from the citadel

An archetypal 'absent-minded genius scientist', and a researcher of portals. His name is Isaac Kleiner, he's aging by the time Combines invaded earth and escaped.