Naruto is often still very popular in fanfictions. Writers like to explore different sides of his character, such as his growth after becoming Hokage in new and unexpected ways.
Sasuke also remains a fan favorite. Fanfic writers might create scenarios where he reconciles with his past in more complex ways or forms new alliances that were not possible in the original story.
Sakura from Naruto might also be popular in these fanfictions. She could potentially have some interesting interactions with Danny, like comparing her medical ninjutsu with Danny's knowledge of ectoplasm - based healing (if there is such a thing in the crossover). Also, Tucker from Danny Phantom might be a popular character as he could be the tech - savvy guy in the Naruto world, coming up with new inventions using a combination of Naruto - world materials and his own tech knowledge.
Obviously, Danny and Dan themselves are very popular. Danny as the main hero and Dan as his dark and complex counterpart are central to many stories. Jazz, Danny's sister, is also popular as she can provide a different perspective and often gets involved in Danny's ghost - fighting adventures in fanfiction.
Often, the main protagonists who are caught in the phantom marriage situation tend to be popular. They are usually complex characters with a lot of room for character development as they navigate this strange relationship. Also, any characters who have the power to break or understand the phantom marriage, like wise old witches or wizards in a magical setting, are quite popular.
Well, the phantom stallion itself is obviously very popular. It's the centerpiece of the fanfiction. People are drawn to its wild and mysterious nature.
Danny himself is often very popular. His dual identity as a normal teen and a powerful ghost makes for interesting stories. Also, Robin, with his acrobatic skills and detective mind, is a favorite. Writers like to explore how he would react to the supernatural elements in Danny's world.
Well, obviously Danny and Vlad are very popular. Danny is the main character in the show and in fanfiction, his adventures and growth are always a focus. His ability to balance his human life and his ghost - fighting duties makes him a complex and interesting character to write about.
Madame Giry can be an interesting character to focus on as well. She has a connection to the Phantom and could be used to explore the backstory of the Phantom and the opera house in a different way in fanfiction. For example, a fanfic could explore her own past relationship with the Phantom or her role in protecting him.
Danny himself is often a very popular character. After all, he's the main character of the original series, so in fanfiction, his adventures are usually central. Sam and Tucker are also quite popular as they are Danny's closest friends. Their relationships with Danny and how they support him in different situations are often explored in fanfiction.
Christine is also a central figure. Fanfiction can explore her feelings for the Phantom and Raoul in more depth. Maybe she has a secret side that she hides from both of them, or she could be given more agency in the story, rather than just being the object of the men's affections.
Sam and Tucker are also quite popular. In the multiverse fanfiction, they can have different roles. Maybe in one story, Sam has some hidden ghost powers of her own and becomes a key player in a multiverse adventure. Tucker could be a tech - genius in one universe and a powerful magician in another, adding new dimensions to their characters.