Okay, time for a longwinded one.
I'm a SquareSoft devotee, and particularly a huge fan of Chrono Trigger, if any of you have heard of it outside my sig. :>
In CT, people and magic are classified under one of four elemental baises: Thunder, Fire, Water, and Shadow. Going along with that, I pegged out Shadow as the one I most resonated with at the time.
As for the second part, I've always felt very feline. But in the last couple years I've concluded that it's not full feline; and besides, calling myself ShadowCat would have people thinking I'm Kitty Pryde. :P Thus the respelling Kat
An' there ya have it.