Constant screen-space-width rim shading
Nice question! I hadn't seen the game myself but this question got me interested so I watched some footage on Youtube.

This one for example gives a good view of it: https://youtu.be/Ktjdg3zgRzk?t=15m46s


A few things I notice:


Hair and right leg: note how only the diffuse lit part has rim lighting.
Left wrist: note how the width becomes thinner at the angle with the hand.
On some other part you can also see the rim lighting is affected by cast shadows too. 


From these observations, I think there are two effects used:


A diffuse shading with a sharp transition between lit a unlit, that happens somewhere below 0 so the light leaks behind a little.
The rim lighting that, as you mention, has an almost constant screenspace width.

From the look of it and some of the artifacts, my best guess is that it's actually some edge detection filter based on depth, which is then used in combination with the diffuse lighting so the rim lighting doesn't affect the unlit parts.


It's a pretty cool effect, thank you for point it out!