Castellan Garran Crowe is both Captain and Brotherhood Champion of the Purifiers of the Grey Knights Chapter of Space Marines. The Purifiers are a group of specialists that burn shit with their minds and are the purest-hearted of the Knights. Castellan Crowe, needless to say, is both of these things, as well as being one of the best swordsmen in the Grey Knights.
Sounds like a Mary Sue, no?
Ironically, Castellan Crowe is the only Grey Knights character (aside from Brother-Captain Stern) that has not earned the enmity of /tg/.
Y He No Part of Ward's Dick Club?
Because he's a badass.
He's a badass because he wields a daemon sword.
A daemon sword called the Black Blade of Antwyr.
A daemon sword that could not be destroyed by the Inquisition.
A daemon sword that mindfucked every other guy that had ever wielded it.
A daemon sword that corrupted three full sectors and required the entire Grey Knights Chapter to step in and stop it.
A daemon sword that actively tries to murder Crowe every time he wields it.
A daemon sword that has near-infinite power, just waiting to be unleashed...
But Crowe just uses it as a regular sword, because using that near-infinite power would corrupt him.
Also, barely enduring the influence of a single daemon weapon makes a lot more sense (and is a lot cooler than) not only surviving in the Warp, but actively destroying all the Chaos Gods' shit.