Malcador the Sigillite

Malcador the Sigilite (Also known as Malcador the Hero) was Regent of Terra, Chief of the Imperial Administration, 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 while the Emperor went crusading. He managed to keep the Imperium of Man running in the Emperor's absence. After 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 Comissars, 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.

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 of 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, and all around awesome bureaucrat.

The organizations he founded--the Administratum, the Officio Assassinorum, and the Inquisition--have been fucking up ever since without his guidance.