Understir is my personal AU, The personalities are completely opposite, while they keep a few of their original traits.
In this AU, Napstablook is confident, outgoing normally, but still doesn't talk too much. Mettaton is insecure and not confident, but still has a robot body and a TV show (In this world, the TV show's purpose is to help him with confidence. If that helps or not is to be decided. Sans is more motivated but still loves puns and sleep. Papyrus is a bit colder (Like underfell but not) but still has a warmer side when you get close to him. Undyne is less confident, hates her position, and just wants to be friends. She still ends up with alphys. Alphys is a lot more confident in her choices, but still the nerdy lil scientist she is Asgore is unwelcoming, but still a good father figure. Toriel is the same as Asgore. Cold, but still a good mother. Flowey is helpful and kind, but still soulless and still Asriel deep inside. Asriel is kinda rude but still emotional. Chara is less sassy and more quiet, but still the kind narrorator (CHARA ISNT EVIL, WE ARE EVIL- THE PLAYER!) Frisk is whoever you want them to be!