You could use inkscape. It is a vector program. I find the best use for it is to make what is called "final lines", after a sketch (or you could google images, find parts of things you like, and cut it up and put it together, using an image editor like gimp), you can use inkscape to make clean outlines. I also prefer to use the shape mode modifier, to change it from simple clean lines, to look more like pen strokes that taper on and off (I also use custom shapes to further improve the natural feel). I'll even do some basic color filling with it.
Afterwards, I'll load the .svg file into blender, which it can animate. The issue with blender is that it is a fairly technical program that is not easy to start using if you haven't tried it before. Sometimes the svg doesn't quite generate the look I want, so I finish painting in gimp, and when I take it into blender for animation, rather than an svg that is already in parts, it is one image, so I have to make a mesh to uv map the image to, and morph the mesh to animate it. wich is fairly technical. something simpler and easier to do this with, would be Live2D but I don't know if it cost money or not.
