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When the developers realized that they would need a character to play off Cave Johnson during his recordings, they created a scientist character named Greg. However, they found it a waste of money to hire a new voice actor for a few lines, and instead economized by cutting Greg and adding Caroline instead. This allowed them to use Ellen McLain as a voice actor and provided an origin story for GLaDOS. According to Cave Johnson, in a line which was also cut from the game, he fought a bear once.
PeTI DLC Edit
In the Portal 2 Perpetual Testing Initiative DLC, Greg is acknowledged but heard only as mumbles in the background. Cave Johnson is heard talking to him. These clips are played throughout courses made in the PeTI DLC. After a few chambers he will tell the player that he, the real Cave (Cave Prime), will say "Chariots, Chariots" in order to identify that it is him and not an alternate Cave talking. Some alternate Caves include a Mantis Cave, a cat Cave, "CaveDOS", as well as several others. It is unknown why Greg is used instead of Caroline.
It is worth nothing that during one of Cave Johnson's sound clips from PeTI, Greg's "Creepy Daughter" gets a hold of Cave's audio equipment, commenting how they'll be testing "Forever and ever," before Cave intervenes and chastises Greg for letting his daughter get in.