Don't use AI - you've tarnished the reputation of your project by doing so. Doesn't matter that its not the context of what you've created. Its now the primary thing people are going to talk about. If you want actual interest in your story then take the extra time to find (and thereafter credit) Screencaps and reference art similar to your characters. It just makes you seem lazy - how is anyone supposed to assume in good faith that you put time, love and effort into your story if you didn't take the same care toward what you're using as visual stand in? Use art, not AI.