Combat Skirt Ready! by ConvolutedPlot
Absolutely amazing robot-suit story by ConvolutedPlot on DeviantArt. It follows Ruby and Penny from RWBY and Penny’s desire to show Ruby her newest features. Turns out, Penny can now act as a kind of suit for people to wear. Ruby, deciding to indulge Penny’s idea, quickly realizes she got more than she bargained for, especially when Penny’s shows her other new ability. Making herself look like other people.
There’s also this (Archive Link) alternative version of the second part, that Gamekid2903 wrote.