Good work Killer B, this is what I had in mind! You can also control the density with the 1-0 number keys on your keyboard.
With density you can start to control your edges. Here's a paintover on your drawing to demonstrate.(the one on the left is the paint-over, the one on the right is the paint-over as well but I lightened it up to write on it
Edges can be soft, hard, and anywhere between the two. You can have razor-sharp edges and ones that seem to disappear. Gradients are also soft-edged.
Brushes are also either: soft-edged or hard-edged, and anywhere between.
In paint tool sai, the brush tool is hard-edged. The airbrush tool is soft-edged.
You can also soften hard edges you've already painted with the 'Water' tool. Use it with light pressure.
Have fun and experiment with this as well. It will bring another dimension to your drawings :).