So firstly, a word of warning. This feature is completely untested. It should work, but a lot of things should work and don't. That said there's no harm in trying to get it to work, if your sprites don't load this time, complain at me, hopefully with an error message, and I'll do my best at getting it to work. Version 188.8.131.52 also had some bugs with the origins, so for that version all sprites but the head are 32px by 32px.
Now, lets take a look at how this feature compares to the other modable features. These types of mods don't go on the modlist, but they still have a name. They go in a folder with the same parent folder as the modlist, named the name of your weird OC. This is the name you will type into the dialogue box. Inside that folder, there will be a mix of up to 4 png and gif files. Any files that are not included there will be assumed to be empty/invisible. You may skip to the TLDR if you plan to just modify the standard sprites, which is much easier than creating them from scratch as detailed below.
Png files should be in strip format, ie however many pixels tall by width * frames. That means that, for example, the arms, will be 32 pixels high and 96 pixels long, with three distinct images in it.
Gif files are simpler since you can just make the frames into a straight up animation.
Either type should work.
The files needed are as follows: (name them just what's in the ""'s)
"head" - 21px tall, 19px wide. Two frames. The first frame is the character icon, which can be a neat picture for the other modes but should be the character using their invisibility mechanic for Katia mode. The second frame is just what the character looks like in-game.
"leg" - 32px tall, 32px wide. 8 frames. Starts in a neutral position, has the left foot forward on the 4th frame, and goes back to neutral on the 8th frame. The game will handle the flipping and mirroring for you. Note that you will have a lot of empty space, since the characters are smaller than 32px by 32px.
"arm" - 16px tall, 18px wide. 3 frames. 1st frame is both arms in neutral, 2nd frame is right arm forward and left arm back, and 3rd frame is left arm forward and right arm back.
"tail" - 3px tall, 15px wide. Base of tail is on the right.
TLDR: Take the files found in this addon, and edit them to your liking. Then rename the folder and put it in the same place you found the modlist in the other tutorials.