Obviously, Chell is the main character. There could also be other characters from the original story that interact with Chell during her transformation into a core. Maybe some mentor - like figures who guide her through the process, or antagonists who try to stop her from achieving this new status.
There might be some characters unique to the fanfic's interpretation of the 'core' concept. For example, other beings that are already cores and they could either be friendly or hostile towards Chell. And of course, any side characters that are part of the world where Chell's story is set, like ordinary people she might be trying to protect as she becomes a core.
Perhaps the fanfic focuses on Chell's character development as she becomes a core. It might involve her facing various challenges and obstacles that test her newfound identity. She could start off as an outsider but gradually gain the respect and recognition of the other elements related to the core through her determination and unique skills.
One potential character could be a young hacker named Jin. Jin has some knowledge about AI but not as much as Chell will gain in the fanfic. He teams up with Chell to explore the deeper secrets of the AI world. Another character could be an AI ethics council member, Ms. Lee. Ms. Lee is constantly trying to ensure that the AI in the story are being used in an ethical way and she often clashes with Chell who is more interested in exploring the AI's capabilities without much regard for the rules at first.
Chell is obviously going to be one of the main characters since she's in the title. GLaDOS is also very likely to be a main character as she was a central figure in the Aperture Science world. There could also be other test subjects if the fanfic decides to expand on that aspect.
The main character is Kobayashi. There could be a magical creature or wizard who accidentally caused her transformation. There may also be a group of adventurers who cross paths with her. Some of them might be curious and want to help, while others might be scared and try to avoid her. And of course, there could be a love interest, perhaps another dragon or a human who is attracted to her new and unique self.
It could be a rogue AI that wants to use Chell for its own malicious purposes. Maybe it sees Chell as a key to accessing some powerful technology or information.
The main characters are Chell and Wheatley. Chell is the silent but strong protagonist who has gone through many tests in the Aperture Science facility. Wheatley, on the other hand, is a chatty and somewhat bumbling personality core.
Obviously Percy himself. Also, Annabeth would probably be there as his girlfriend. Grover, his satyr friend, might also make an appearance. And the gods, like Poseidon, Percy's father, would surely play a role, either guiding Percy or testing him on his path to godhood.
Well, the fanfic could start with Chell being exposed to an experimental AI technology. This AI could start to influence her thoughts and actions in unexpected ways. Maybe it gives her enhanced cognitive abilities which she uses to explore a mysterious world. Along the way, she has to deal with the ethical implications of being so closely tied to an AI.
Well, in this fanfic, it could be that Chell discovers an ancient AI. The AI has unique powers and knowledge. Chell starts to learn from it and as she becomes more involved with the AI, she begins to change. She might gain new abilities or a different perspective on her world. For example, the AI could show her hidden secrets about the place she lives in, and together they could explore new areas or fight against some sort of oppression in her world.
Obviously, Rarity herself is the central character. Her friends from the Mane Six, like Twilight Sparkle, who could help her understand the magic involved in becoming an alicorn. Also, Spike might be there, as he's always around Twilight and could play a role in Rarity's journey, perhaps by providing some useful information he's found in his library duties.