Greg

Greg is, during the events of the Perpetual Testing Initiative, the assistant of Aperture CEO Cave Johnson.

Originally, he was written to appear during Portal 2's single-player campaign, however he was replaced by Caroline prior to release and was considered to be cut from the game before being added back as Caroline's replacement in the Perpetual Testing Initiative DLC.

Overview
According to the The Final Hours of Portal 2, one of the reasons Greg was cut was to eliminate the cost of hiring a new voice actor that only played a small role, instead reusing Ellen McLain to voice Caroline.

Trivia

 * In a line cut from Portal 2, Cave Johnson reveals that Greg had fought a bear in the past.