Roboute Guilliman

The Primarch of the UltraSMurfs. He crash landed on MacCragge as an infant, and as a result of the trauma, suffered massive brain damage. It is said, of the primarchs, only Guilliman was retarded. He did, however, have a very large ego, and demanded that everyone who talked to him call him their spiritual liege. That being the only entry requirement to join the Ultramarines legion at the time, many thought it a fair trade for power armor. During the Horus Hersey, he was tricked by the promise of a shiny penny into deploying his legion into the middle of nowhere by Horus, and came back to find everybody else was dead. The few scattered space marine legions that were left were in no condition to protect the Imperium. After a hard day's thinking, Guilliman realized that all existence rested on his shoulders. Which explains today's Grimdarkiness.He quickly grew confused by this effort, and sat back to making a coloring back, now known as the Codex Astartes. However, its brilliance as a book is only because Creed ghost wrote it for him. He was killed by Fulgrim, or at least, should have been killed, but Guilliman has yet to realize that poison kills you, and so he spends his days frozen in place, trying to remember which ninja turtle is which.

Matt Ward had a hard on for Guilliman, so he went all historical revision on his ass, and made him into the genius we think we know today