[The spirit doesn't really show all that much in the way of expressions, does she? Or at least she doesn't reveal all that much easily; then again, neither do Tui or La; it's not like she's had in depth interactions with the spirit koi.]

The oath I swore to my people... and to Tui and La. I'm one of the priestesses of the Northern Water Tribe charged with protecting them in the Spirit Oasis near the North Pole.

[Granted, she's not at the North Pole now, and perhaps that's worth questioning, but that's the answer to the question the Mother of Faces asked.]