Ortan Cassius
"He knows a lot about killing aliens, which is no bad thing." - Asmodai giving a rare compliment to his counterpart in the Ultramarines.
Ortan Cassius is an absolute badass.
He is also, coincidentally, an Ultramarine. Luckily, Cassius is so badass that he is immune to Matt Ward's vile influences. However, he cries manly tears that the rest of his Chapter has devolved to such faggotry at the hands of Ward.
In all seriousness, though, Cassius is the Master of Sanctity and head Chaplain of the Ultramarines Chapter. He also happens to be their oldest active member. He made his name during the First Tyrannic War, in which he attempted to save the northern polar fortress and led efforts to reclaim it after the loss of the First Company. This did not go well, and Cassius was (nearly) crushed to death by a rampaging Carnifex. However, he was such a badass that he somehow clung to life while the Apothecaries rebuilt him, Six Million Dollar Man/Deus Ex: Human Revolution style. When he woke up, he had a new goal in life: exterminate the Tyranids, which he had come to believe were the Ultramarines' punishment for being such faggots. To accomplish his goal, he gathered the Tyrannic War Veterans, the only other badass thing the Ultramarines have ever produced.
Why is Cassius So Badass?
You may have noted a disturbing trend in Ultramarines special characters. Marneus Calgar is the BEST CHAPTER MASTER EVAR, Varro Tigurius is the BEST LIBRARIAN EVAR, Torias Telion is the BEST SCOUT SERGEANT EVAR, Antaro Chronus is the BEST TANK PILOT EVAR, and Cato Sicarius is the BEST SECOND COMPANY CAPTAIN EVAR (okay, maybe not so much on that one). In other words, they're all a bunch of fucking Mary Sues. They have essentially no character traits besides being THE BEST EVAR. There's nothing setting them apart from other Space Marines besides the fact they're just better. They're so absolutely perfect that even Fulgrim himself weeps bitter tears that he can never be as perfect as the Ultramarines special characters.
Cassius, on the other hand, is not the BEST CHAPLAIN EVAR; he's not even necessarily a good Chaplain. But he has an interesting backstory that ties into the Ultramarines' fluff as a whole, he epitomizes the dogged determination that characterized the Ultramarines pre-Ward, he doesn't have plot armour, and he makes both his faction and the faction he fights look good. For instance, he doesn't impale a Carnifex on its own scything talons; he gets torn to shit by it. This makes Tyranids look like a credible threat for the Ultramarines, unlike the poor Daemons, which just look impotent next to the Grey Knights.
In other words, he's everything Matt Ward's characters usually aren't. Whether this is because his background was already set in stone by the time Ward got his hands on it, or it's a fluke of nature, the world may never know... but at least the world can breathe easy, knowing there is a least one Ultramarine that isn't a complete and utter faggot.
He also patronizes Calgar, which is worth the price of admission alone.