Logan Grimnar
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.
Grimnar is also the only non-Daemon entity to slay a Grey Knights Grand Master. Yes, you read that right, he fucking decapitated a space marine that hunts Greater Daemons for a living... But you are probably thinking "Wait...Hold up. What the hell did a Chapter that has never fallen to Chaos, and is regarded to have geneseed directly from the Big Guy himself, do to earn the scorn of Grimnar so much that he slays one of their Grand Masters?" The answer kiddies is that the Inquisition strikes again, in retarded hindsight ordering the Knights to fire on the Wolf Wolf Ships during a *cough*peace meeting*cough* probably wasn't the best idea. Naturally, the Great Wolf being the Great Wolf, flies up to the Inquisitorial ship, walks right up to the Grand Master that gave the order and cuts his fucking head off then shoots down four Grey Knight Justicars for the lulz. Long story short and much rage-only-the-screwing-up-of-the-inquisition-can-do later, Bjorn the Fell Handed had to finally step in and kick some Wolf and Knight balls in equal measure to remind them trolling around in 40k will not be tolerated. So everybody goes home pretty fucked up, but with their heads on straight (well except for the one grand master).
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.
He was probably created from the combined geneseed of both Leman Russ and Eddard Stark.
(A little known fact is that Fenris is actually the same world as the setting of a Song of Ice and Fire, thousands upon thousands of years later. They are just in their longest-ass winter yet.)