Dante wishes he was this cool... or old.
Logan Grimnar is Chapter Master of the Space Wolves, and all-around cool guy. He has been the Space Wolves Chapter master for over 700 years and has been kicking ass even longer, making him one of the oldest Chapter Masters currently serving. He has shown great battle prowess and his thirst for blood rivals that of their mighty Primarch, Leman Russ. As such, he is said to be the greatest living warrior in the Imperium. (At least until the Spiritual Liege Matt Ward gets his hands on him and nerfs him down below his favored sons the ultrasmurfs, until then suck on that Marneus Calgar) Grimnar wears a pimped out suit of terminator armour equipped with a storm bolter and carries his trusty axe Morkai (which he got from beating the shit out of a Chaos champion and stealing it from him, then beating the corruption out of the daemon weapon until it submitted to him) into war with him. As of the 5th Ed. Space Wolf Codex, he also has the ability to buff his squad with some fun tricks, give all nearby allied models an extra attack for one turn, AND makes Wolf Guard into a Troops choice, allowing you to pimp out your whole army. Isn't he generous? Well, he's certainly generous to the enemies of the Imperium - WITH THE ASS-KICKINGS!
The Ecclesiarchy does not like him, and has charged him on multiple counts of treason and heresy for being a decent person to the Imperial Guard and other non-Astartes organizations. Like after the Armageddon War (in which he actually managed to make Angron even ANGRIER), when those assholes in the Inquisition decided to purge all the Guardsmen who fought there because they were "tainted" or some shit and Grimnar called them out on their dickery. As a result, the Inquisition keeps clear of Fenris whenever it can, for fear of what might happen. Yes, you read that right. The Inquisition has no fear of the heretic, the xeno, or the daemon, but Logan Grimnar? He fucking terrifies them.
When that fucking failure Abaddon kicked off the 13th Black Crusade, good ol' Creed knew just the person to call. See Grimnar is an old drinking buddy of the clever Cadian (which says a lot about Creed's stamina, as Space Wolves only drink Fenrisian), and when the massed Legions of Chaos came knocking on the Cadian Gate he knew the Old Wolf would have his back. Grimnar rocked up and ran the show, combining his testosterone fuelled manliness with the brass balls of Creed to put the Armless Wonder back in his place. He was named Supreme Commander of the Imperial Forces, which included Ecclesiarchy and Inquisition Forces, who took it about as well as you would expect.
And he also shares the same resentment towards the Dark Angels as the rest of his chapter (Where's the fluff on this, exactly? He probably does, though. Bathrobe-wearing pansies.). Also among his hatreds is that of the Galactic Partridges, who have successfully managed to trollingly capitalize on his badassery for at least a few centuries now. Despite their dickishness, Logan still manages to maintain the bulk of the credit for his mighty deeds.
He is the only known furry that you can think is FUCKING AWESOME without being HERE-BLAMMED.
No relation to Wulfrik, Garmr Hrodvitner, Valmir Aesling, Sven Bloody-Hand nor any other Norscan.
 Logan Grimnar and the Inquisition
Logan Grimnar is known to border between the line of loyalist and traitor in the eyes of the Inquisition and the Administratum, particularly for his actions after the First War for Armageddon. The Inquisition thought that everyone but themselves and the Space Marines who participated in the war were tainted by Chaos. In addition, the Administratum didn't want people to find out that the Imperium was weaker than they said it was, since Angron's forces had decimated Armageddon's population and cities. So they ordered the PDF, all the serving guardsmen, and civilians of Armageddon to be sterilized and rounded up into work camps to slave for the rest of their short lives while Armageddon is re-populated.
This action horrified the Chapter Master and he ordered his forces to protect all the departing Imperial Guard troop carriers with his fleet before the Inquisition and Administratum could purge them out of paranoia and pretentiousness respectively. This action naturally pissed-off both groups, especially the Inquisition. While not in open war, some very bad blood existed between the three parties, more specifically with Inquisitor Lord Ghesmei Kysnaros, the Inquisitor responsible for purging the old population of Armageddon. Eventually, the Inquisition, under the guise that they wanted to bury the hatchet, decided to negotiate a truce with the Space Wolves in neutral space....then the stupid started to commence.
The Inquisition, along with the Grey Knights, opened fire on the Space Wolves' fleet, with the Space Wolves loosing 4 ships and having 1 crippled. Apparently, instead of trying to reach a mutual understanding, the Inquisition thought that it was a better idea to force the Space Wolves to surrender and force them to take a crusade of penance. You can imagine how Ol' Grimmy took this. So, he was teleported into the Grey Knights command ship, under the guise of negotiating a surrender, and then proceeded to chop off Grand Master Joros' head for firing on his fleet, along with 4 Justicars, just for the lulz.
The Inquisition, who's not really one to admit a mistake or apologize to anyone, decided to attack Fenris itself with the Grey Knights, the forces of the Ordo Malleus, and the Red Hunters chapter, a chapter who has close ties with the Inquisition, all with the intent of forcing the Wolves to heel to the power of the Inquisition and force them into a penitent crusade for their action. Yes, we're dead fucking serious, this slaughter on a massive scale is because of the Inquisition's ego trip; the Administratum didn't get involved because, while just as arrogant as the Inquisition, they're nowhere near as brazen or influential militarily. The Great Wolf, not one to back down from a scrap, especially with asshats like Kysnaros, responded in kind.
Despite having the Grey Knights, which alone should have ensured victory, the Space Wolves managed to take the blunt of everything and actually repulse the assault. During the fighting, Loggy and a force of 20 Terminators, attacked Inquisitor Lord Kysnaros' ship and killed him. But in the end, the Space Wolves could not hope to beat out the forces of the Ordo Malleus and an entire Space Marine chapter at the same time, so Bjorn asked Logan if he could cut the whole vendetta thing with the Inquisition and seek for peace, especially since he got the lowly Inquisitor who started everything anyway, which he did. Somehow, after much negotiations, the Space Wolves and the Inquisition settled everything and broke off the current conflict. They didn't declare the Space Wolves heretics and the Space Wolves decided to stop beating everyone down.
To this day however, Ol' Grimmy still hates the Administratum and the Inquisition for their actions on Armageddon; the latter in particular for their altercation. The advice of Ulrik, Grimmnar's oldest friend, keeps him from giving a reckoning to the Administratum. While the Inquisition takes great care not to get on the Space Wolves' bad side anymore, lest they risk a war with one of the only loyalist chapters who has the balls to give them the middle finger and say: "Fuck you, we do what we want".