As a guideline, the character sprites have borders around them, so I suggest selecting the bucket/fill tool, setting the colour to transparent, which will remove a selected colour, and holding shift to remove all of the selected colour, and then drawing your character. However, it only works on computers, unfortunately.
12/1/23 TEMPORARILY PUTTING THIS ON HOLD TO WORK ON SOMETHING ELSE (hopefully I actually finish it at some point :skull:)