Restrict Espers by Character

I would do it myself, but I haven't got a clue on how to do it.

You could use @Subtraction code. Copy all the code block into a text file and replace the extension by .asm. Then use xkas 0.06 in command line like this: xkas filename.asm rom.smc. You'll need to edit this line to a free space block in bank $C3 ($03F091-$03FFFF) that TWUE doesn't use (if you look it up in a hex editor bytes would be a bunch of FF).

Code:

org $C3F091         ; Start of free space in bank C3.

This is one problem for sure, I know TWUE use $C3F091 ($03F091) and up so if there is room it would probably be near the end of the bank (nearer $03FFFF). Note that you'll need to make sure code doesn't overflow in bank $C4. There could be more compatibility problems with the hack, but that one is one for sure.