Most likely Naruto himself. Since it's in the Naruto universe, he's often the central figure. His friends like Sasuke, Sakura, and Kakashi could also be main characters as they usually accompany Naruto on his adventures.
The main characters would probably start with Naruto. He's the protagonist of the Naruto series, and any fanfic set in that world often revolves around him. However, it could also heavily feature his closest comrades. For instance, Sakura might play a crucial role if it's a more relationship - centered day trip, perhaps dealing with her feelings for Naruto or Sasuke. Kakashi could be there as a mentor figure, guiding the younger ninjas through the challenges of the day trip. And there's always a chance that new characters created specifically for the fanfic could also become main characters, depending on the direction the author wants to take the story.
Well, Naruto is a shoo - in as one of the main characters. But it could also focus on side characters like Rock Lee or Shikamaru. Maybe it's a day trip organized by Shikamaru, and he has to deal with Naruto's exuberance and the others' quirks. And of course, depending on the plot, antagonists like Orochimaru or some new villain could also make an appearance during this day - long escapade.
Since I'm not well - versed in 'the day tripper novel', I can't name the main characters. However, it's common for novels to have a protagonist who is at the heart of the action. In this case, it could be the day tripper themselves. There may also be supporting characters like friends or family members who either encourage or oppose the day tripper's excursions, but without reading the book, it's all just speculation.
I don't know for sure as I haven't read it, but it could be the day tripper themselves as the main character. There might also be other characters they interact with during their day - long adventure, like a fellow traveler, a local guide, or someone they meet at the destination.
Naruto himself would likely be a main character. Since he is the protagonist of the Naruto series, the Groundhog Day concept would be interesting to apply to him. He could be the one stuck in the time loop, trying to figure out what's going on. Sasuke is also a likely main character as he has a complex relationship with Naruto. His reactions to Naruto being in the time loop and how he might be involved in breaking it could be a big part of the story.
Well, it could be a fanfic that focuses on a day in the life of Naruto or other characters in the Naruto world. Maybe it's about Naruto going on a day trip to a neighboring village, meeting new people, and getting into some sort of trouble or learning something new. It could also be about his friends joining him and how they interact during this day - long journey.
Well, obviously Naruto himself would be the main character. There could also be his friends like Sasuke and Sakura, who might be involved in the interactions between the two Narutos. Maybe they are confused at first by seeing two Narutos, but then they help them figure out what's going on.
It could still be Naruto himself. Since he's the central character in the original series, many fanfics would likely continue to build on his story.
Obviously, Naruto would be one of the main characters as the story is about him being the son of Whitebeard. Whitebeard himself would also be a major character, even if he might not be around for the whole story. And then there would be the other members of the Whitebeard pirates, like Marco. They would play important roles in Naruto's life in the pirate world, either as mentors or comrades.
Obviously, Naruto and Jiraiya are the main characters. They are the focus of the 'naruto meets jiraiya' concept. Naruto, the energetic and determined ninja, and Jiraiya, the wise and powerful mentor.
Naruto is surely going to be one of the main characters. After all, it's a Naruto fanfic. Sasuke is also very likely to be there as his rival and friend. Sakura might also play an important role.
Naruto himself would probably be one of the main characters. Since he has always been about protecting his friends and the village, a 'greater good' narrative could put him in situations where he has to make even more difficult decisions regarding the well - being of everyone. He might have to sacrifice his own desires for the greater good.