Draw natural illustrations for concepts and comics, with Krita
Krita is a free powerful illustrator that you can use for concept drawings, comics, etc. Krita started as a painting app, a GIMP competitor, then focusing on illustration.
The program’s interface resembles Adobe and can be somehow “overwhelming”, but if you play a while you won’t face problems using anything you need.
Krita’s brushes support filtering, duplication, pixelation, and many more functions, even letting you save profiles. Layers and masks are of course supported, and a series of effects.
It’s impossible here to describe completely this powerful application, but since it is free, just download and see for yourself if it is able to help you.