I'd lean toward Fey, also.
Shall we base the stats on anything (an Incorporeal Dryad? a Shadow?) or build it from scratch?
Well, they aren't incorporeal, but gaseous. We've got precedent for that. Just gotta find 'em.
EDIT: That didn't take long...
Gaseous (Ex): A moor hound's insubstantial form grants it several benefits. A moor hound doesn't actually fly, but floats just above the ground. As a result, it is not subject to ground terrain effects, such as ice or a grease spell, but it is subject to winds as if it were a flying creature. It cannot wear armor, manipulate solid objects, or enter water or other liquids. It can pass through small holes or narrow openings--even mere cracks. It also can occupy squares occupied by enemies.
While we're arguing over that, what are we doing for its attacks?
The original beastie has two 1d4+2 damage attacks, but doesn't say what they are. I'm thinking we should go for a "bite" for one plus a "claws" for the other.
As for the damage, I'm wondering whether we should make them both 1d4 with a +2 enhancement bonus? Giving them the 14-15 strength needed for a +2 damage bonus doesn't seem right for a wispy Small-sized fox.
I like it!
What about the gaseous form question?
I'm thinking something like:
"Gaseous Form" (Ex): A kitsuni kasumi can assume gaseous form at will (as the spell, except it can take the run action while gaseous and uses its X ft. land speed). The kitsuni can take gaseous form as an Y action or return to solid form as a Z action. Its gaseous form is almost indistinguishable to its solid form. It requires a ? check to determine whether a kitsuni kasumi is solid or gaseous.
My brain's a bit mushy right now, but I can't find anything dreadfully wrong with that at the moment. Although if it remains on land, I'd add the bits about floating above the ground from the early gaseous ability.