Roboute Guilliman
Roboute Guilliman (A.K.A.: Rowboat Girlyman, Rawbutt Girlyman, Robot Gulliver, Robot Gorillaman) is the Primarch of the Ultrasmurfs. He crash landed on Macragge as an infant, and as a result of the head trauma suffered massive brain damage and severely inflated his ego which only Commander Dante could rival. 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 Heresy, he was tricked by Horus with the promise of a shiny penny into deploying his legion into the middle of nowhere, and came back to find everybody else on Terra was dead, where his only comment upon arriving was "oops".
The few scattered space marine legions that were left, were in no condition to protect the Imperium as their ranks were either dead or defected to Chaos, so the Ultramarines was virtually the only chapter left to supplement the other chapters. After a hard day's thinking, Guilliman realized that all existence rested on his shoulders....we don't know exactly why but that's retardation for you, which explains today's Grimdarkiness. He quickly grew confused by this effort and sat down and made a coloring book, now known as the Codex Astartes. Its brilliance as a book only came about because Creed went back in time to ghost write it for Roboute, adding the now-familiar organizational scheme, which divided the Space Marine Legions into Chapters of more manageable size, and tactical doctrines for every situation, like the formidable Steel Rain.
One of his exploits was killing Alpharius (Or Omegon.... Or a standard Alpha Legion Space Marine.... We really don't know), the Primarch of the Alpha Legion, after taking the time to break-off from his stringent rulebook and think tactically for once. Alpharius wasn't a particularly hard fight since the Alpha Legion didn't embrace Chaos and so didn't have his dick pumped up with the ruinous powers, so he was still only as powerful as a normal Primarch, unlike Angron and Fulgrim who were mad powerful already with their Chaos pacts. However, he didn't count on the Alpha Legion to still keep fighting, even after the death of their Primarch, and so the Ultramarines had their asses handed back to them.
Much later, his throat was sliced by Fulgrim with a poisoned sword which should have killed him, 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 in a stasis tube, trying to remember which ninja turtle is which.
Also, since you are left fully conscious within a stasis tube, experiencing the last thing you felt before you went under... Yeah, daaaaaaaang.
Matt Ward has a hard-on for Guilliman due to daddy issues so he went all historical revision on his ass, and made him into the genius we think we know today as written in the 5th Space Marine codex.......that's just for canon terms anyway, the denizens of /tg/ still think of the Ultrasmurfs and anyone in it as they did in previous codex, which is to say like dicks.