His companions like Amy Pond and Rory Williams are often involved. They are important as they have been with the 11th Doctor through many adventures and their reactions to his regeneration are crucial parts of the story.
Well, Naruto himself is obviously a main character as it's about his regeneration. Sasuke might also be involved as his rival and friend. Their relationship could be explored in new ways with Naruto's regeneration, like Sasuke being jealous or worried about Naruto's new power.
Obviously, the 12th Doctor himself is a central character. Then there's Clara, his companion. Their relationship is often explored during the regeneration process, as she might be there to witness it or even have some influence on how it unfolds. Another common character could be the TARDIS. It's an important part of the Doctor's life, and in fanfiction, it might react in unique ways during the regeneration, like the console lights flickering in a special pattern.
It really depends on the source material. For example, if it's based on a superhero franchise, it could be the main superhero like Superman or Batman. If it's a fantasy - based one, perhaps a wizard or a chosen hero from the original work. There's no one - size - fits - all answer as different fanfictions can be based on different universes and thus feature different popular characters.
In many fanfictions, the 10th Doctor's emotional journey during regeneration is complex. He not only has to deal with the physical pain of the process but also the emotional turmoil. He may be afraid of what the new version of himself will be like. There are also memories of his past regenerations flooding back, adding to his distress. And his relationships with his companions play a huge role. He might worry about how they'll react to his new form, and whether they'll still be by his side. All these emotions are often explored in great detail in fanfictions.
The main character would surely be the 10th Doctor himself. His companions, depending on the era, like Rose Tyler or Martha Jones, could also be main characters. They play important roles in how the Doctor adjusts to his new form.
The Doctor's companions are also very popular. For example, Yaz. Fanfic writers like to explore her character more deeply, her backstory, and how she reacts to different situations in the TARDIS. They might also show her developing new skills or having personal growth during her travels with the Doctor.
One interesting idea could be exploring the Doctor's internal conflict during regeneration. He might be grappling with memories and emotions from his 12th incarnation. For example, his relationship with Clara could play a role in how he approaches regeneration, perhaps having visions of their adventures together and feeling reluctant to let go of that part of his life.
One interesting idea could be exploring the Doctor's internal conflict during the regeneration process. He might be grappling with memories of his past selves and the adventures he had. Another could be how his companions react to the sudden change. They may be in shock or try to help him through the difficult transition.
Edward and Bella from 'Twilight' are often used in difficult love fanfiction. Their relationship has elements of danger and the struggle between Edward's vampire nature and Bella's human existence. Another pair could be Harry Potter and Hermione Granger. Some fanfictions explore a more romantic relationship between them, which is difficult considering their strong friendship and the overall plot of the original series.
Doctor Strange could be a popular character in a boggart crossover if it's with the Marvel universe. His mastery over magic could lead to some really interesting interactions with boggarts. He might try to use his spells to either contain or study the boggarts, which would create exciting storylines.