My thought was Lion. Or maybe even Leo. A fairly common nn for Katherine is Kat (at least, I'm hearing it everywhere lately! Much more often than Kate, Katie, Kit/Kitty, or any of the other options!), and cat to lion makes a nice transition, imo. I think Cathan, Thane, and Aaron are lovely alternatives, too, though. Maybe you could use initials?
My first thought was Kit too...
I was reading the blogs yesterday about nicknames... and Kit is the British nn for Christopher, but also emerged as a nickname for Katherine. Kristopher if you're into creative spellings.
But a Christopher called Kit would work for me. though I like the above suggestions of Thane and Aaron.