Rein is known to often give assessments of the progress of his company and gives a monthly update in the magazine Game Developer where he provides a short update on the state of the Unreal Engine. He resides in Raleigh, North Carolina, though he was raised in Toronto, Ontario, Canada.
Rein first got involved in the video game industry when he got in touch with John Romero, who at that time worked for id Software. Since Rein was a fan of the previous Commander Keen games, Romero got him to playtest the then under development Commander Keen 4. Rein was then brought in to handle the business side of id as its "probationary president". He then negotiated a deal with FormGen to publish a retail Commander Keen game. This business relationship with id lasted until Spear of Destiny (which again, he negotiated as a retail version of Wolfenstein 3D). He was later let go from id during the development of Wolfenstein 3D, after a difference of opinion with the rest of the staff. He was also later joined at Epic by Jay Wilbur, id's ex-business manager, and Rein's involvement in id did mean that he was briefly mentioned in the book Masters of Doom.