Definitely Alice, as she is at the core of any 'Alice in Wonderland' - based story. The Caterpillar could be another important character in this kind of fanfic. The Caterpillar, which is already a symbol of transformation in the original story, could have a more complex role as it observes Alice's slow shrinking. Maybe it could offer some sort of guidance or be a source of knowledge for Alice as she deals with her changing size in this new version of the story.
Well, in this type of fanfic, the slow shrinks can enhance the exploration of the strange and illogical world of Wonderland. As Alice shrinks slowly, she may have more time to interact with the various characters she encounters. For example, her conversations with the Cheshire Cat or the Mad Hatter could be more in - depth and revealing, as she is changing size gradually and they might react differently to her continuous shrinking.
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.
Well, perhaps the shrinking is a result of some magic gone wrong. As she shrinks, she loses her confidence at first. But then, she realizes that her new small size gives her an advantage to explore hidden corners of Wonderland. She could find secret passages or solve mysteries that were impossible for her at her normal size.
Well, there could be a little mouse. Since Alice is shrunk, the mouse could be her guide in this new, small world. It might show her where to find food or shelter.
The Cullens are often involved too. Edward, for example, as Alice is part of the Cullen family. Their interactions can be explored in different ways in fanfic, perhaps showing a different side of their family dynamic.
The Bandersnatch. It's a strange and fearsome creature that has its own set of rules and behaviors that make it stand out.
Naruto is often involved. Since he's the main protagonist, writers like to put him in situations where he has to deal with deception, either being deceived or trying to deceive the enemy. Sasuke is also a common character in such fanfics. His complex personality and his past actions make him a great target for deception plots or a character who could be doing the deceiving.
Ben is obviously the main character as it's a 'ben 10' related fanfic. His various alien forms are also important characters in a sense. For example, Upgrade might be involved if the story has to do with his feet being able to attach to and control machinery in a unique way related to his feet. Gwen could also be a part of the story, perhaps helping Ben understand some strange phenomenon related to his alien feet forms.
Kakashi can also be involved in some shukaku naruto fanfic. He could be Naruto's mentor in understanding the Shukaku better. He has a wealth of knowledge about the ninja world and could offer advice to Naruto on how to handle the power of the Shukaku and any potential risks associated with it.
There could also be antagonists like Hawk Moth. Since he is the one sending the akumas, he would be involved in the backstory of Marinette's akumatization. He might have a particular reason for targeting Marinette this time, which could be explored in the fanfic. And perhaps some new characters could be introduced as a result of Marinette's akumatized state, like other heroes or villains that are drawn to the chaos she creates.