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.
Well, it could be about Kobayashi suddenly finding herself in a situation where she starts to transform into a dragon. Maybe it's due to a magic spell gone wrong. She then has to deal with newfound powers and instincts.
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.
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.
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.
Hank would be a very likely character. He already has a complex relationship with Connor in the original story. If Connor starts to become deviant, Hank's reaction could be very interesting. He might be suspicious at first, but then as he sees the positive changes in Connor, he could support him.
Obviously, Natsu would be the main character as he is the one turning into a dragon. His friends like Lucy, Gray, and Erza are also likely to be main characters. They would play important roles in either helping Natsu deal with his transformation or being affected by it in some way.
Tohru is a very prominent character. She is strong - willed and has a deep affection for Kobayashi. Her attempts to blend into human society while still being a dragon lead to many interesting scenarios. Kobayashi is the ordinary human who gets caught up in this extraordinary situation. She is often the voice of reason. Kanna, on the other hand, is cute and innocent. She is like a child in many ways and her interactions with the other characters are really sweet.
Obviously, Naruto would be one of the main characters. He's the protagonist of the Naruto series, so it's very likely he'll play a central role in the fanfic. Sasuke could also be a main character as he often has his own story arcs that intersect with Naruto's. And maybe Sakura too, since she's part of the main trio. There might also be some new characters introduced specifically for this fanfic related to the Dragon Blade, like a mysterious ninja who has a connection to the blade's history.
There could be new characters introduced, like old family retainers of the noble family Harry is now part of. Maybe an old wizard who serves as an advisor to Harry on matters of the lordship. And of course, some antagonists such as rival lords or wizards who don't want Harry to gain too much power.
Tohru is often paired with Elma. Their dynamic as two dragons in the human world is interesting. They can be rivals at times, especially when it comes to their different views on humans and how to interact with them. Another common character paired with Elma is Kobayashi. Since Kobayashi is the main human character, her interactions with Elma provide a lot of comical and heartwarming moments, like when Elma tries to understand Kobayashi's work - related stress.