A great paint tool, the full licence fee could put people off after the trial has finished. Be aware that this hasn't been updated since 2016.
Paint Tools SAI is a paint program that was specially designed to facilitate manga creation. Suitable for both beginners and advanced artists, Paint Tools SAI has a very large range of tools, including superimposed layers, vector graphics, watercolor, sketch, painting, and more.
You can use the software for a trial period of 31 days, after that to continue to use this software, you must purchase a license from the official site, which costs around $50.