Here are some settings that simulate COPIC markers in Paint Tool SAI. Enjoy!
To add this tool: 1-Right Click on the BRUSH tool. 2-Select DUPLICATE 3-Apply all of the settings shown in the screen shot.
Rename the brush to COPIC if you wish. ^^
UPDATE: has kindly provided me with a screen cap of an alternate translation of SAI's tool set so the settings according to his go as follows: BELOW the 2 brush texture and shape settings,, Blending ->Color Blending Dilution ->Opacity Mix
Under ADVANCED, the 3 check boxes on my screenie are: Dens ->Density Size ->Size Blend ->Clr