Sorry to say this, but if you think that you can create the characters and backgrounds in Ren'py itself, then you are in for a rude awakening. You will need to create the characters outside of Ren'py, with programs like Blender, Poser, Daz Studio or something similar, OR pay an artist to draw characters for you. The backgrounds are made in the same way.
Ren'py simply gathers everything and puts it together into a single program, so that it becomes an actual game. Think of a game, as 2 pieces of fabric. One piece of fabric is visuals (characters and backgrounds) and the other is story/text. Ren'py would be the string that is used to stitch them together into a single unit, but you need to have the 2 pieces of fabric already there.