Personally, I think if you want to get Mary a reputation for doing well without becoming a youkai or relying on other external help, you're best off not getting an enchantment at all. The first one seems half-assed to me for both getting the most out of being enchanted when/if the time comes, and wanting Mary to prove herself as a plain human first and foremost. That's what I really don't like about it.
Overall, still leaning towards "it sounds like a good idea, but I think I'll give it a few years first because damn" as the ultimate solution. If we get one, though, I'd prefer to go the whole hog. But that doesn't have to be now or in this Quest. Once Mary and Renko are good and ready and have tried out the outstanding-plain-human thing.
Actually, giving that a chance for a while to see whether Kilga's idea works out and agreeing on doing the full thing if it goes poorly could be a satisfactory solution for all of us. If it works out, cool - there's always later if things fall apart. If it doesn't, then there's a good alternative to help out that hasn't shown any overwhelming downsides, and we've gotten evidence that turning it down hasn't worked out perfectly.
