Agent Six, also known as Six, is a Providence agent, and Rex's handler. His name comes from the fact that he is the sixth most dangerous man on the planet. Due to Six's strict by-the-book personality, Rex goes as far as to refer to him as a "nanny, just more aggro".
He is currently operating against the reformed Providence, whilst trying to regain six years worth of memory that he lost.
Six has an extremely stoic personality, often keeping a stern look on his face. Despite his emotionless exterior, he is a caring person deep down. An example of this is when Six first discovered Rex, who at the time was a young boy, trapped beneath a collapsed building. After Rex confessed that he was scared and alone, with no memory of ever having a family, Six showed sympathy for him and took him back to Providence. It is implied that Six was not given the title of "One" (or any higher) because of the softer side of his personality, which made him less of a threat than the others.
Six rigidly believes that orders are orders, and that they must be obeyed even if one doesn't like them. However, Agent Six isn't without morals. For example, when White Knight was going to use a Bleach Bomb to stop zombie E.V.O.s from crossing the Brooklyn Bridge, Six removed the detonation key, effectively disobeying White Knight's orders. In an attempt to cheer Rex up, Six offered to buy Rex a few more days in Cabo Luna, despite the vacation not being authorized. Six also offered Rex the choice of leaving Providence. He also tricked White Knight into thinking he, Rex and Doctor Holiday would return on The Keep.
Six has also stated that he dislikes firearms, which is why he prefers to use a sword in combat.