Why exactly? Because I am always a fan of the large and silent types.
That and being an anthropomorphic fox shinigami is also something going pretty nicely for him.
The greatest thing drawing my fandom to him is just the many questions about him that I have, With some examples being "Why is a fox?", "How did he live before becoming a Shinigami?" and "Would he eat dog treats if given the chance?".
Of course that last one being just a bit out there and perhaps a future subject of an omake chapter for him.
And who doesn't love the idea of a Shikai that is the disembodied sword, Hand and arm of a black armored giant?
It's obviously got a great suprise factor in a fight and even in Bankai, A giant in armor staring you down isn't something you just laugh off...Unless you are Aizen.
Finally, That one scene before he got owned by Aizen was just awesome as who could ever expect that giant fox man to just appear behind you screaming and attempting to lay the smack down upon you?
Edit:Little bit of a grammar fix. (Darn me and my love of pro-wrestling.)