Cypher is said to be a Fallen Angel. At the end of the Horus Heresy, Lion El'Jonson returned to Caliban to find Luther turned to the forces of Chaos. Jonson landed and took up the fight against Luther while the Dark Angels fleet bombarded the planet from above. Eventually, Luther killed Jonson but when he did, he realised his mistakes and collapsed, unwilling to fight any more. The Chaos Gods realised they had failed and sent a vast warp storm that sucked up the traiter Dark Angels and spread them across time and space.
Cypher was one of these Fallen Angels but no one actually knows his true name. He appears randomly throughout the galaxy, but is rumoured to be moving towards Terra. He is famous for his pistol abilities, being able to fire precisly on every shot and even firing two weapons at once. His characteristic combination of Bolt Pistol and Plasma Pistol are known throughout the galaxy as every Segmentum has rumours of Cypher like people. It is likely that Cypher has appeared in every Segmentum over time, although a good deal of the rumours of his appearance are probably other people in similar clothing.
He wears his Power Armour but covers it in robes in an attempt to hide himself, but this look has made him obvious. Perhaps this was the point, but he does seem to attract other members of the Fallen Angels to his side. It is unknown how they communicate and why they are drawn to him.
The final distinguishing feature of Cypher is his sword. It is a mysterious weapon that he has never been seen to draw on the battlefield. Some rumours say it has some link to the Dark Angels ancient history, some say it is broken. Most agree that it is probably a sacred artifact of the Dark Angels.
One source, named the Heresy of Cognitive Dissonance by Cardinal Elyass Vallkante, mentions some form of knife used possibly by Cypher. It is said he struck a being of golden light and his weapon was absorbed into the being. It is possible, then, at some point Cypher carried a C'tan Phase Weapon.