Anime girls are not 2D.

Anime girls are represented as \mathbb{R}^2 . All their properties can be expressed with one or two variables, or using a one or two-dimensional vector. They can be represented in an euclidean space as a plane.

However, they have physical properties that suggest that they might have more than two dimensions. For instance, while they can only move left or right, they can also move in a "depth" axis, and when they do their size scales up or down depending on the direction of this "depth" axis. This looks similar to how objects look when they move on a 3-dimensional space.

This indicates that, while not noticeable for the 2D girls, there is indeed a third dimension.