Dr M was the main and final villain of Sly 3: Honor Among Thieves. Dr M used to be a member of the original Cooper Clan, but betrayed them in attempt to steal the fortune from the Cooper Vault. Before his betrayal, Dr M worked with Sly Cooper's dad, Conner Cooper and his friend, Jim McSweeney. While the gang were still together, Dr M served as the technical specialist, comparable to Bentley's role in the modern Cooper Gang. Dr M, Connor and Jim amassed a great deal of fortune, and stored most of it in the Cooper Vault.
Over time, Dr M started to feel like he was just a sidekick to Conner Cooper. This angered Dr M as he felt undervalued, and motivated him to leave the Cooper Gang and set up shop on Kaine Island. It was on this island where the Cooper Vault was located. Dr M began to start his quest to break into the Cooper Vault, as he felt the fortune was righfully his.
Many years passed, and Dr M still had failed to successfully break into the Cooper Vault, despite using numerous resources and strategies. As time went on, Dr M became more and more paranoid, eventually creating a secure fortress around Kaine Island, trying to prevent any interference. It is said that the level of security that Dr M set up was equivalent to that of Fort Knox.
Later on in Sly 3, Sly and the Gang make their way to Kaine Island to confront Dr M. A fight breaks out between Sly and Dr M, resulting in Sly losing his cane. Dr M at first had posession of the cane, then seemingly gave it back up to Sly. Dr M had secretly placed a tracking device on the cane, so that when Sly went into the Cooper Vault, Dr M knew which path to follow him through to reach the end of the vault.
After Sly defeated Dr M in a battle inside the vault, Carmelita Fox made a surprising appearance. Dr M took a shot at Carmelita, but Sly dove in the way, taking the hit (Sly later on proved to recover just fine). The Cooper Vault started to crumble, as Carmelita and Sly evacuated. However, Dr M refused to leave, since he had spent his life trying to get into the vault. The walls then collapsed on Dr M, ending his reign of terror.