Rogal Dorn
Rogal Dorn is the Primarch of the Imperial Fists Chapter, which was a Legion before Horus fucked everything up. He is also regarded as being kind of an asshole towards his siblings. He's also the 40k Abraham Lincoln because he has a bitchin' moustache and can't tell a lie.
The Imperial Fists are quite famous for fortifying the shit out of everything and INTENSE CAMPING which pisses off every other race. They are highly likely the reason that the Emprah didn't get raped by Horus during a certain (totally awesome) siege. His evil twin/Brother is Perturabo, Primarch of the Iron Warriors, who turned to Chaos because he was one of Horus's ass slaves. As Perturabo constantly fucks up all of Rogal's shit, Rogal has developed a special kind of rage towards him. His being an asshole wasn't limited to just Perturabo, though - he also had problems with Corax, Curze, Alpharius, Russ, and.... Really anyone who wasn't the Lion, Sanguinius, or Guilliman. In the case of Alpharius, him and Guilliman being a pair of giant assholes is what eventually led to Alpharius saying "fuck that" and signing up with Horus.
During the Siege of Terra, Rogal went with the Emprah and Sanguinius to Horus's battle barge to face the traitor. When the Emprah totally fucked up Horus, Rogal seemingly came out of nowhere and saved his ass, then got all pissy about it because he was only a few seconds short, and found Sanguinius dead and the Emprah dying. Surprisingly, he did not turn into an Angry Marine, but still whines about it like some emo bitch who can't move on, which Corax purportedly found painfully ironic.
When Roboute Guilliman showed off the Codex Astartes and demanded that the Space Marine Legions break up into Chapters, Dorn lead the opposition against it, and only submitted after those cowards opened fire on his fleet, deciding that tradition really wasn't worth starting another civil war over.
Much later, after the Second Founding, he declared to everyone that he'd capture his nemesis, Perturabo, inside a metal cage. Pert then trolled Dorn by setting up the "Eternal Fortress", which was fortification after fortification that led into a fortress that Perturabo was residing in. It took the Imperial Fists three weeks to wade through the shit Perturabo set up. However, it was too late for Dorn to realize that he has just been trolled by Pert. When his Imperial Fist strike force reached the actual fortress, it was actually empty - a giant, centrally-open kill-zone with virtually no cover - and with the Iron Warriors waiting in ambush. Suffice to say, the Imperial Fists then got their asses handed to them, and wound up having to use their fallen brethren for cover up until the Ultrasmurfs came in and rescued them. By the time they escaped the planet, the casualties that the Fists had taken were enormous.
Soon after this, Rogal got in a big-ass ship and got pwned by Angron in the first Black Crusade, (he did manage to wreck the whole damn crusade though) therefore proving that RAEG ends all things and how Angron is among the only Chaos Primarchs that actually gets anything done these days.
| The Primarchs of the Space Marine Legions |
|---|
 Loyalist Corvus Corax - Ferrus Manus - Jaghatai Khan Leman Russ - Lion El'Jonson - Roboute Guilliman Rogal Dorn - Sanguinius - Vulkan |
 Traitor Alpharius/Omegon - Angron - Fulgrim Horus - Konrad Curze/Night Haunter - Lorgar Magnus the Red - Mortarion - Perturabo |