I designed my game Arena Of Prometheus to be somewhat straightforward to modify. The “playerAttributeArrays” function has a ton of arrays that store stats for each of the characters (eg. damage, hp, animations). Each character has a respective index in the arrays. So for the first character, Truffle, all of their data is stored in the index 0 of each array.
You could add another index to each array to make another character. Also add another selection box to the title screen to select your new character. Then go to the on button pressed blocks and make their moves. You can check which character is selected with “player#Selection” variables.
If you want a challenge, add another character! If you have any questions just ask.