Personally, I think a Lawful Good Pit fiend should not exist. Pit Fiends aren't just evil... they're [Evil], the very incarnation of Evil. Is a pit fiend evil? Is water wet? You shouldn't be able to redeem a pit fiend - it's not a human with morales and forgiveness, it can't be sorry, it can't feel remorse. Pit Fiends can no more be Lawful Good than an elephant can fly. It's just impossible.
That, and over elaborate character designs like this always seem way too far fetched. I mean... being guided by a good spirit, sure, cool. But why does it have to be a redeemed pit fiend? Why not just a good spirit of some sort? And if you really want her to register as minor evil (which, I admit, would be kinda cool), why not *really* have it be because of all the evil spells she's been the target of?
As for the stats... why do you need them? The pit fiend is incorporeal and can only influence her subtly, right?
-The Souljourner