Oh, Owly!
I've met Andy Runton at a couple of cons and he's super-nice. He uses these very expensive brush pens for Owly and the other things he does, and he's a true artist. I am in awe of him. Owly is Copyright (and TM!), Andy Runton.
Correction from Andy: "I just use a normal (synthetic, no sables hurt) brush and india ink when I draw Owly. I only use the brush pens at the shows. And they're not THAT expensive. ;) "