Flowey - Flowey has ripped black petals and an unsettling smile. He attempts to warn Frisk/Chara the mistake that they have made and how to fix it but before he finishes his sentence he gets burned to death by Toriel. The most creepy thing about Flowey is that he melts like Undyne when Toriel kills him for unknown reasons.
Toriel - Toriel is very different from her Undertale counterpart, she takes attempts to murder Frisk/Chara and murders Flowey the instant she sees him. In battle if you tell her that Flowey was her son and show evidence she will use her strongest attack on herself, killing her in an instant. She usually tells Sans sadistic jokes about death and suicide. Also, the top and the bottom of her dress is ripped, leaving only a big band of cloth around her body and her sleeves.
Napstablook - Napstablook looks like Underfell's Napstablook, but he is incredibly depressed, even more deressed than he normally was in Undertale.
Sans - Sans' smile is wider than Undertale Sans', his bones are completely black and he wears a zipped up ripped jacket with blood stains. He always has his hood up, which keeps changing colour. Sans has a deep hatred for Papyrus and they rarely see each other. If you look Sans in the eyes his eye sockets will slowly deform and if you look away he will seem like nothing has happened. Sans, like Flowey is also aware of what happened. He hates you with all of his heart and he is also now indeed the weakest character, with his attacks being a very short bone that can be easily avoided.
Papyrus - Papyrus hates everyone and everything, he has lost all emotions and he speaks in lower case. His teeth are missing because they have been knocked down by Undyne one by one because he refused to follow her rules of killing humans. He doesn't have a moving animation just like anyone else. Papyrus is way stronger than his Undertale counterpart because his LV is higher. He also wears a blue scarf, blue boots, and blue gloves. He also has sharper shoulders, longer hands, and longer boots.
Undyne - Undyne is Captain of The Royal Guard and is very abusive towards Papyrus by calling him weak and hurting him and her skin is red. Other than that she is the same.
Temmie - Temmie is still Temmie.
Alphys - Alphys has no mouth or eyeballs, instead she has black holes instead of eyes. Undyne is the only person that she can still feel emotions for and she thinks of the others as garbage, she also made the determination experiment go wrong because she was too "busy" with Undyne.
Mettaton - Mettaton never transforms into his EX form or NEO form, but you can defeat him by breaking his glass LED lights on his face. He also is made of bronze.
Asgore - Asgore is the same as his Undertale counterpart except that his clothes are red and black. He has the urge to kill Frisk/Chara but he refuses to do so and goes insane. If you show him a picture of him Asriel and Toriel together he will fall to his knees and cry.
Chara - Chara is still a murderous demon except they tell Frisk to not kill or else something horrible will happen. If Frisk doesn't listen to Chara they will both be erased forever making the monsters suffer for eternity. Also, due to the fusing between Chara and Frisk, Chara now wears a green shirt with purple stripes.
Frisk - Frisk is the one who committed the many genocide routes and caused Chara to fuse with them, it is unknown if they wanted this to happen or if it was just a mistake. Also, due to the fusing between Chara and Frisk, Frisk now wears a blue shirt with one big tan stripe.