the dev is pretty iffy about including someone else's characters as they don't want to deal with any potential drama it could cause (e.g. characters potentially being pulled out or people feeling like they're left out entirely), on top of the dev preferring for the game to be wholly original; the game will only feature characters directly made for it as a result, though the dev's pretty open to other artists making characters specifically for the game like with theano or chateau
do keep in mind though that as a possible alternative solution, any external character can always be implemented as a modded character if you really want to see them in-game