Do you want to make your own mod that adds unique costumes for a specific character? Then here's a small tutorial on how to achieve it!

Step 1: Create your own mod
Go to This PC > Program Files (x86) > Steam > steamapps > common > The Binding of Isaac Rebirth , open the mods folder and create a new folder with the title of your mod.

Step 2: Enable Unique Costumes
Go back to the The Binding of Isaac folder, open the resources-dlc3 folder and copy the players.xml file. Return to your mod folder, create a new folder called resources-dlc3 and insert the copied file into the folder.
Open the file (Notepad++ required) and choose a character (e.g. Cain). Create a parameter called costumeSuffix and add a name to it (e.g. costumeSuffix="cain" ).

Step 3: Add Unique Costumes
Create a new folder called gfx , enter it and add another folder called characters . Enter it and add yet another folder titled costumes + the name you put in the costumeSuffix parameter (e.g. costumes_cain ).
Now add your spritesheets including your character-specific costumes into that folder.

Please regard that some costumes are located in different folders, therefore:
If you want to add unique Rebirth / Afterbirth / Afterbirth+ costumes you need to put them into resources > gfx > characters > your costumes folder.
If you want to add unique Repentance costumes you need to put them into resources-dlc3 > gfx > characters > your costumes folder.