David Johnston wrote:

Simply being half-human isn't necessarily the pivotal difference.
After all, I doubt that Shippou or the the flea-youkai have that
kind of evil aura...

Never said that his being half-human was why his aura was different. 
He's just the first youkai that Kaede has seen without a solid evil
aura.  But then, Kaede also hasn't been around youkai much at this
point. ;)

Krista
-- 
"Knowledge is just opinion that you trust enough to act upon."

                         from -Children of the Mind-
                         Orson Scott Card