A hard watercolor brush for shading.
This is my basic shading brush when working on big illustrations.
Here a small tutorial how I use it:

Add the basic shades with this brush. Either use a blur tool or go over some edges again with very low pen pressure to create soft vs hard edges.

Use the default CSP airbrush to add a few more subtle shades and highlights.

Use this brush to add bounce light to your character.

The last step I do is adding hard lights on the side of the light to make it pop a little more.

Hope you enjoy this brush!