Gimp, Photoshop, or any editing software that has access to the channels tab.

You will need to render it two times. The normal render, and a render with no lights and a white greenscreen background.


So open up your editing software. I am using gimp. Open the first render, and import the second render as a seperate layer, and make sure it's the top layer.





















Go into your channels tab, right click any of the channels and click channel to selection.




















Go back to layers, and delete the 2nd render. Now add a layer mask to the 1st render, and click selection.


The character will probably be the only thing transparent, but don't worry. Just select the layer mask, go to colors tab and hit invert.


There you go! Now you have a transparent render of your character.