Here's what you need to do. Have an extracted romfs folder, and unpack the garcs 1/3/3 and 0/2/1. Once they are unpacked, you should be able to easily drag the Kris models+textures into the folders, and then rebuild the garcs, delete the folders and you are set to play as Kris instead of May. If you want to overwrite a different character, you can open the files inside the folders with Ohana3ds, and find the number of the character that you want to overwrite. Then you can just rename it, drag it in, and done.