It could also be a new character, a powerful monster that has been hidden away for a long time. Alice's presence in the Undertale world awakens this monster, and it sees her as an intruder and decides to stop her from leaving or causing any changes to the Underground.
Another option could be a corrupted version of Asriel. Maybe Alice's arrival somehow stirs up the darker parts of his past and he becomes a force against her, believing she is a threat to the delicate balance in the Underground.
It could be a new vampire who is jealous of Bella's relationship with the Cullens. This vampire might have some special power that allows them to manipulate memories, like Alice's memories of Bella. They see Bella as a threat to their own plans in the vampire world, so they try to create chaos by making Alice forget about her.
The Caterpillar could be the main antagonist. He has always been a bit of an enigma in Wonderland. Maybe as Alice shrinks, he sees her as a threat to his control over the small part of Wonderland he rules and tries to stop her from exploring further.
A jealous student could be the main antagonist. This student might have a crush on Alice and see Bella as a threat to their relationship. They could start spreading rumors about Bella, trying to make her look bad in front of Alice and the other students.
Perhaps a group of survivors who have become so desperate to get out of the Borderland that they are willing to sacrifice anyone. They could be the antagonists as they would clash with the main characters' more moral approach to survival.
Voldemort could be the main antagonist. He might see Alice's vampire abilities as a threat and want to use Harry to get to her. He could also be interested in using her powers for his own dark purposes.
It could be another monster who is jealous of Sans getting all this new - found attention as he turns into a human. This monster might try to stop the transformation process or make Sans' life difficult in his new human form.
It could be a powerful chef who has been corrupted by the desire for ultimate power in the cooking world. This chef might see Naruto and Alice as threats to his dominance, especially if they start to gain popularity and win Shokugeki battles. He could use underhanded tactics like stealing their recipes or sabotaging their ingredients.
It could be a power - crazed human general. He might have a deep - seated hatred for monsters, believing them to be inferior. He ignores all the attempts at peace and drives his troops into battle against the monsters, determined to wipe them out.
It could be a new vampire who has a vendetta against Bella for some reason. Maybe this vampire wants to take over the territory or is jealous of Bella's relationship with the Cullens.
It could be the Queen of Hearts from Alice in Wonderland who has somehow gained new and dark powers. Maybe she wants to conquer not only Wonderland but also Naruto's world. She could use her army of playing cards and newfound magic to create chaos.