Agent Six, simply 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."
After a six month gap and Rex's disappearance, he was operating against the reformed Providence and Black Knight, whilst trying to regain six years worth of memory loss.
Little is known about Agent Six's past. He was homeless at some point in his life, and during that time One took him in and trained him. He later joined The Six, a group of dangerous mercenaries led by One. A failed mission in Malaysia broke the group up and Six went to work with White Knight. 
Shortly after the Nanite Event, One began to turn EVO and it is implied that this prompted White and Six to join Providence. While working for Providence, Six helped save innocent lives by fending off various EVOs and put his remarkable fighting skills to good use. He later met Doctor Holiday, who assumed he was just another hired gun who believed that killing EVOs was the only option to rid of them. However, he altered her impression of him when he stated that he believed killing EVOs wasn't right, just as she did.
During a difficult mission, Agent Six discovered and rescued Rex, a cure for EVOs, and introduced him to Holiday. After they attempted to keep him secret, White interrogated the truth out of Holiday, found out about Rex, and attempted to dissect him on a molecular level to reverse-engineer him along with Doctor Fell.
Six fought for Rex and eventually won him over. Throughout his years there, he was promoted as one of Providence's top agents and positioned as Rex's handler.
He later revealed to Rex that there was a time when he wanted to be the number One dangerous man in the world, and that he was well on his way until he met Rex. Six believed that his work for Providence was more important than The Six's work, saying that "fighting a planet-wide plague ranks just a bit higher than breaking arms and legs..."
Agent 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 EVOs 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.
After losing the last six years of his memories, Six acts more like the mercenary he once was. Six became more aggressive, cocky, merciless, irresponsible, sarcastic, and a bit of a loner. He is much more flirtatious towards Dr. Holiday and toward Five, to a lesser extent. After deciding to try to become the man everyone knew, Six started to act more responsibly, but still displays tendencies from his "mercenary" side. This is seen later when he becomes irritated and interrogates an innocent man at sword point. It was also proven when he jumped at the chance to use fatal moves on the Black Pawns when he learned that they were just machines. However, Six mentioned the belief to hold back when battling humans.
Six wields a pair of magna blades that can slice through most materials with ease. He stores them in his sleeves and when equipping them, they quickly spring out and unfold. He is an expert swordsman shown to be adept in wielding his magna blades as two swords, a double-bladed staff and as a single katana; being a formidable opponent when battling with either of them. Six has never used guns in combat as he corrected Dr. Holiday, who mistook him as a hired gun.However, he did once use a gun as a melee weapon but never pulled the trigger.
With swords such as these, they have various capabilities. These swords can be used as a powerful magnet when joined together, forming a shape similar to a tuning fork. When using this method, Six can manipulate metals to fall on his opponents. On one occasion, he also used this method to help innocent civilians by using light poles to prevent the collapse of a small bridge. Six is also proficient in the art of knife throwing, as shown when he tosses dart-like blades from very far ranges or angles.
Six has peak human agility, speed, reflexes, endurance and strength. His skills were clearly seen when, after jumping out of a Providence carrier, he unsheathed his swords and performed a series of acrobatic feats. He sliced through boulder-sized chunks of debris in midair, jumping from each of them, so that they would not fall onto the civilians below— all before he delicately touched the ground. Even once while locked in a closet by Rex, Six kicked it without much effort, breaking open the door that was entirely made of metal. Six's reflexes are fast enough that he can deflect point blank gunfire with a single katana and can even deflect them in any direction he wishes. He has even demonstrated being able to throw swords through Breach's portals and accurately target her while she is traveling inside them.
Six is also skilled in various forms of martial arts and has been able to defend himself without his swords in numerous battles. With fighting moves as skilled as his, Six can throw powerful punches and kicks to beat his enemies. Agent Six has been seen to throw back large opponents, like Peter Meechum, while in combat.
Six appears to be a pilot, having his own Providence jump jet that is dark gray. He has his very own hover board he uses for solo transportation. He is also very knowledgeable of Providence's rules and codes, most likely because of his by-the-book personality. As implied by Rex, Six also has considerable knowledge on things that can harm others due to his previous nature as being one of the world's most dangerous people. As a result of his prior affiliation with The Six, Agent Six also has many connections to things others do not.