I will definitely need to try some of your tutorials. Your style is so beautiful and I can only hope to reach 1/4 of the skill you have at it. What kind of brushes do you use if you don't mind me asking?
You can make an ink pen by right clicking an empty box in the tool menu and selecting pen/pencil and then changing the settings. I don't remember what else you had to do, but if you search for "how to make: ink pen for paint tool sai" you should find a small tutorial of the subject.
But it's not the ink pen, tested it already... and KyouKaraa is lying about the tools, because none of these make the "stringy brush" effect you can see very visibly on the brown-haired girl in picture number three, also in the other pictures, or the outlining effect.
I tried to search for something, but no, nothing :/ I have been trying to make this effect for a long time.