Greg

Originally written to appear during Portal 2, Greg was a scientist and a friend to Aperture CEO Cave Johnson. Though what role Greg may have played is unclear, he was replaced by Caroline before the release of the game. Therefore, Greg may have been an assistant to Cave.

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.