How to unlock: Talk to Royse near Stalwart Hot Springs (M1,6).
South of Hot Springs, you will encounter a woman named Royse. After you talk to her, she tells you that she is looking for her daughter that has gone missing. Of course, you need to help her find the girl.
You can ask around the village, or go straight to where Mylla is. She is in the Whitestone Hollow, in the Cave (M6,3). She will be surrounded by several witches. There are two things that you can do here.
The easiest thing to do is simply to attack the witches. They pose no challenge for your party. Then, several animal spirits appear and you will have to defeat them as well - they won't go down as easily, but there's nothing to worry about. After you win the fight, return to Stalwart Village and talk to Royse (M1,6). You will be rewarded with some experience points.
Alternatively, you can convince Mylla to go back to her mother of her own volition. For this, you will need either 12 or 14 (depending on the chosen dialogue option) Intelligence points. You can also convince her by giving her the Darkest Rauatai Cookies. If you've managed to avoid the fight, you will receive a small gift from Mylla after you return to Royse.