She's a cgi 3d model with effects applied. Go look at the new version of berserk to see a more obvious version. They do it because they can just motion pose the models and only have to illustrate the background. It's easier and cheaper but not as good as full 2d.
The new character designs are much closer to the original designs from the light novel. She's supposed to look fairly young. Iirc she's described as looking like a teenager.