Malcador the Sigillite
Malcador the Sigillite (Also known as Malcador the Hero, Malcador the Mary Sue and Malcador the Laundryman Of Teh Emprah) was Regent of Terra, Master of the Administratum, Grand Master of the Officio Assassinorum, and all around bestest bud of the Emperor of Mankind during the Great Crusade and the Horus Heresy. Having served the Emprah during the unification of Terra, Malcador remained as Regent of Terra, acting as a prime minister who kept the day-to-day activities of Imperium of Man running while the Emperor was absent due his Great Crusades. After the Emperor returned to Terra and set up the Council of Terra, the Emperor appointed Malcador to the position of First Lord of the Council while he set to work on the Webway. When the Horus Heresy broke out, Malcador was appointed head of the Inquisition and charged with the formation of the Grey Knights.
He was the most powerful psyker of his time (other than Big E and Magnus the Red), being able to do things such as plunging the entire moon of Titan in the Warp to protect what would become the Grey Knights from the attacking Traitor Legions. He also invented Chaplains, though back then they were more like Commissars, keeping order and looking for heresy, instead of the modern day priests. There is also a tank named after him, though nowadays another tank named after an important hero and leader of the Imperium is used instead, the Leman Russ, passing over yet another perfectly servicable vehicle. Russ is a glory hogger like that.
He is dead. Very, very dead. During the siege of Terra, Malcador's psychic powers forced him to take the Golden Throne while the Emperor fought off the Chaos Space Marines. Since he wasn't a psyker on the level of the Emperor, though, the process wore him out, and he crumbled to dust after trading places with the Emperor. With his last ounce of strength, he allowed the Emperor to communicate with his mortal servants one last time. Truly, an all around awesome bureaucrat.
The organizations he founded--the Administratum, the Officio Assassinorum, and the Inquisition--have been fucking up everything else and each other ever since without his guidance.