Coming on June 7th, the mod tool will allow players to change the appearance of characters, enemies, weapons... And of course, it is integrated to the Steam Workshop, which means you will be able to upload your creations to the Workshop. There are some rules you have to follow, though, just to keep it appropriate.
However, you're will not be able to create your own stories and you custom characters, at least not yet. Hiromichi Tanaka says: "Players can create their own little locations, dungeons, things like that, and play around with the assets we provide".
So this means that you will be able to only modify the existing characters and elements. It is nothing huge, but still a neat thing to be added. However, I am pretty sure that when the modding tool is released, some folks will find a way to create your own character.