Suzaku could be a main supporting character. His relationship with Lelouch is complex, and in a fanfic involving Charels, he might get caught in the middle of their power struggles. C.C. is also likely to be a key supporting character. She has her own goals and powers, and her influence on Lelouch would play an important part in the story.
The Knights of the Round could also be main supporting characters in this fanfic. Their loyalty to Charels and their power within Britannia would create a lot of tension. They could be used by Charels to try and stop Lelouch's plans, or some of them might defect to Lelouch's side depending on how the story unfolds.
Nunnally could be a significant supporting character. Since she is Lelouch's sister, her presence could add an emotional element to the story, especially if Charels' actions affect her in some way. Also, Kallen might be there as a supporting character. Her loyalty to Lelouch and her skills as a pilot would be important factors in any plot involving Charels and Lelouch.
In some fanfics, a great plotline is when Lelouch is forced to work undercover for Charels for a time. This would put him in a really difficult position, having to balance his own goals of overthrowing Britannia with the tasks Charels assigns him. He might have to make some really tough decisions that could either expose his true intentions or help him gain more power within the system.
I highly doubt it. Given the disturbing nature of the theme (if it involves rape as the title suggests), it's not likely to be popular among the general audience who prefer positive and ethical content.
Since the idea involves such a disturbing concept as rape, it should not be promoted. Fanfics are supposed to be creative works within the boundaries of legality and morality. There's no value in exploring the details of this kind of 'fanfic'.
C.C. is often a main character that interacts with Lelouch. Their relationship is complex and in the fanfic, they might continue to explore their connection, whether it's about the Geass or their personal feelings. Suzaku also could be a key character. He might have to come to terms with Lelouch being alive and they could either become allies again or have a more strained relationship.
The main characters are obviously Lelouch and CC. Lelouch is a complex character with his own goals and abilities, and CC is a mysterious figure with her Code. Their relationship forms the core of this fanfiction concept.
Hermione would likely be a main supporting character. She would be there for Harry, giving him advice about his relationship with Cedric. She might also help them with any magical research or problems they encounter. Ron could also be a supporting character, perhaps at first being a bit confused or skeptical about Harry's relationship with Cedric, but eventually coming around and being supportive, like at their wedding.
Leafpool could be a main supporting character. As Squirrelflight's sister, she could offer her medical knowledge and emotional support during the pregnancy. Bramblestar would also be important. As Squirrelflight's mate, he could be there to protect her and help with preparing for the kits.
The main character is Lelouch Lamperouge. He is the one who drives the story forward with his intelligence and the power of Geass. Another important character is C.C. She has a special connection with Lelouch through the Geass power. Nunnally, Lelouch's sister, is also crucial as Lelouch's actions are often motivated by his love for her. And there's Kallen Stadtfeld, who has her own feelings for Lelouch and is an important part of the story's events.
A family member, like his sister, could also be a key supporting character. She could have a different perspective on Alexander's situation, perhaps initially not understanding what he's going through but then learning more about eating disorders and becoming a strong advocate for his recovery.
Hermione would likely be a main supporting character. She could help Harry in coming up with ways to educate Tom and keep him on the right path. Also, Ron might be there, providing some comic relief and also acting as a sort of brother figure to Tom, showing him what a normal family - like friendship could be like.