The two cousins are obviously the main characters. They could be very different in personality. One might be more adventurous and impulsive while the other is more cautious and studious. There could also be some side characters like an old time - traveler who accidentally sent them on their time - traveling journey, or a love interest in one of the time periods that causes more tension between the cousins.
Well, the cousins are at the center. One might be a tomboyish type who is good at physical challenges, and the other is a girly - girl who is more into social status. In their time - travel adventures, they may encounter a helpful historian in one of the eras who gives them clues about how to control their time - travel. And there could be a villainous character in another time period who tries to take advantage of their catfighting to achieve his own evil goals.
The main characters would be the feuding cousins. One could be the popular and outgoing type, always getting attention, and the other is the quiet and resentful one. Additionally, there might be a wise old figure in the past who tries to teach them the value of family while they are busy catfighting. And perhaps a mysterious figure in the future who is somehow connected to their time - travel situation and helps them resolve their issues.
Well, perhaps the cousins start off with some petty rivalry in their present time. Then, through some strange time - travel event, like finding an old family heirloom that's a time - traveling device, they get sent to different eras. In the past, they might compete for the attention of a historical figure, which escalates their catfighting. As they travel through time, they gradually realize that they need to work together to get back to their own time, and in the process, they mend their relationship.
The main characters would probably be the ones from the original 'Book of Life' like Manolo, Maria, and Joaquin. Since it's a fanfiction, Adelita Cousins might give more focus on one of them and their personal journey.
Boruto is very likely to be one of the main characters. As the son of Naruto, he has a strong connection to the original story and would be a natural choice for a time - traveling adventure. Sarada, being Sasuke's daughter, would also be a key character. Her skills and her relationship with Boruto could drive the story forward in many ways.
Since I don't know the specific story, it's hard to say. But based on the title, the main characters are likely to be the cousins involved in the war. There might also be other supporting characters like their family members, comrades in arms, or enemies they face during the war.
The main characters would be Naruto for sure. He's the center of this time - travel concept. But we can't forget about the other members of Team 7. Sasuke, with his brooding and powerful nature, and Sakura, who was developing her skills. Also, characters like Iruka - sensei, who was important to Naruto's early development, might also play significant roles. And there could be appearances from other ninja like Shikamaru and Choji, who were Naruto's friends and comrades during his genin days.
Naruto, of course. Since they are in the same team in the original story, it's very likely that Sakura will interact with Naruto. She might try to change his fate or help him in a different way. Sasuke is also a main character she'll interact with. Maybe she'll try to prevent Sasuke from going down his dark path. And Kakashi, as their teacher, will also be involved in their interactions.
They are likely to interact with Naruto's past self. It would be an interesting dynamic as Naruto tries to explain things to his younger self without messing up the timeline too much. And Himawari would be there, perhaps teaching her young father some of her own jutsu.
Sasuke's relationship with Sarada could also be affected. If Sarada experiences things in the past related to Sasuke's family or his past actions, it could make her understand her father better. Maybe she'll come back with a new understanding and they'll have a more open relationship.
Batman himself is a likely candidate. Given his intelligence and determination, he might stumble upon a time - traveling device or technology while investigating some strange phenomenon. He would be driven by his need to protect Gotham, perhaps by preventing a future catastrophe that he has foreseen.
Ban could be another one. He has his own backstory related to the sins and his immortality. He might time travel to find a way to break free from his immortality curse or to go back and correct a mistake he made when he was under the influence of one of the sins, like greed when he stole something important in the past.