One use for Artbreeder is to "breed" any two images together (such as a cat and an octopus, which was the first thing my daughter did). So you could upload "parent" faces and create "children" - but in my case I just wanted to visualize my book characters.
Creating faces is not entirely under the user's control - you can't tweak a nose exactly the way you want because you're mostly adding or subtracting "genes" from different parts of the world. But you can easily get photorealistic portraits with a few twists of the dials.
So, here are my results - just my impression of how the characters look, for my own amusement. Most of these characters are related to each other one way or another, so for many I started with the same "base" and fiddled with the age, gender, hair and eye color, etc.
[Mild spoilers ahead for the later books in the Wynter Wild series.]
Let's start with the three Heathcote-Brown sisters from Vermont: Miriam, Patricia, and Katrina. I tried to convey their different personalities - the cool serenity of Miriam covering for the evil acts of her past, the friendly hostess Patricia, and the shy but well-put-together Katrina.
At the ashram, Wynter met these two Aussie boys when they were 16:
Let me know what you think. I'm particularly happy with Patricia for some reason - she looks so kind and trustworthy. Her life is already complete, but here's hoping she adds true love before the series is over...
Don't forget to check out the Wynter Wild Wiki if you want to learn more about any of the characters.