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.
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.
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.
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.
One great 'Wheatley x Chell' fanfic is 'The Unlikely Bond'. It beautifully explores their relationship post - Portal events. The author does a wonderful job of showing Wheatley's growth and his newfound understanding of Chell, and how Chell might come to see him in a different light.
Redemption is also a big theme. Given Wheatley's actions in the game, a lot of fanfics focus on his journey to redeem himself in Chell's eyes. This can involve him sacrificing something for her or proving his loyalty in difficult situations.
One interesting plot could be Chell and Wheatley going on an adventure through uncharted areas of the Aperture Science facility. They might encounter strange and new test chambers that challenge their friendship and survival skills.