Aw I love Arietty! I think it's totally usable - it's close enough to other "legit" names as to not sound totally out there.
I'd probably pair it with something short and more "common".... (Actually, if I were to use it for real I'd probably consider NOT using a MN - I think it strong and unique enough to stand on it's own. I love a lot of names so I usually want to throw in as many as a can for the middle spot but with Arietty I'm really loving the idea of it as a standalone)
Arietty
JuneArietty
LaurelArietty
Paige...