In the episode titled "Promises, Promises" it shows rex's past. He was a giant half machine EVO. He was beaten and six took him back to providence as a little boy. Dr. Holiday wanted a cure for EVO's rather than destroying or containing them. Six presented rex as the cure. Providence tried destroy him for research but he shut down a machine and accidently purged White Knight of all nanites. He then lost his memory again and providence took him for training.