Tsuna and Iemitsu are the main ones. Tsuna because it's his class and he is the protagonist. Iemitsu because he brings in an outside and important element. However, Reborn could also be a significant character. He might be observing from the sidelines, guiding Tsuna even in this situation where his father is involved. His presence would add a touch of mystery and guidance to the story.
Obviously, Tsuna and Iemitsu would be the main characters. Tsuna is the central figure in his class, and Iemitsu is his father. But also, some of Tsuna's classmates like Yamamoto, Gokudera and Kyoko could play important roles. They could react to Iemitsu's presence in different ways, adding more flavor to the story.
In this fanfiction, perhaps there is some sort of danger that threatens Tsuna's class. Iemitsu arrives just in time to help. He uses his skills and knowledge to protect the class. Along the way, the students get to know him better and Tsuna has to deal with the complex relationship he has with his father while in the context of his school life.
The main characters are Tsuna and Fon. Tsuna is often portrayed as a character with great potential that is gradually being unlocked, while Fon is a more experienced and wise figure.
Obviously, Bakugou would be one of the main characters as the story focuses on the encounter with his future self. And also, Deku would likely be a main character. Their relationship has always been a central part of the story, and seeing how Deku reacts to future Bakugou would be interesting. Additionally, Ochaco could be a main character as she has her own unique relationship with both Bakugou and Deku, and her presence could add some different dynamics to the interactions in the fanfiction.
There could be an ancient evil force that has been awakened. This force is interested in the power of the Vongola Rings and wants to use it for its own dark purposes. Tsuna and adult Reborn would need to combine their strengths, with Tsuna's determination and Reborn's combat and strategic expertise, to defeat this powerful and mysterious antagonist.
It's really hard to say who would be stronger. Tsuna has his powerful Dying Will Mode which gives him enhanced strength, speed, and combat abilities. However, Ichigo has his Bankai which is extremely powerful as well. If we consider the environment and the nature of the threat they face, it could change things. For example, if the enemy is more spiritual in nature, Ichigo might have the upper hand as he is more experienced in dealing with spiritual beings. But if it's a more physical battle, Tsuna's training in hand - to - hand combat could make him a formidable opponent.
One possible reason could be that Iemitsu has been absent for much of Tsuna's life. In the story, this absence might have made Tsuna feel abandoned, leading to hatred. For example, Tsuna could have faced many difficulties growing up without a father figure, and all the pain and struggles could be directed towards Iemitsu.
In some cases, the relationship could be developed through the eyes of other characters. Other people in Tsuna's life might tell her stories about Iemitsu that are not flattering, which would influence her feelings towards him. And as she experiences her own interactions with Iemitsu, these pre - conceived notions could turn into full - blown hatred.
Obviously, Mitsuki would be one of the main characters. From Class 1A, Bakugo could be a central figure as he is her son. Other students like Midoriya could also be important. Midoriya's inquisitive nature might lead to some interesting conversations with Mitsuki about hero origins and family influence on becoming a hero.
Tsuna would likely develop a sense of wariness towards Xanxus. He might start to train harder to be able to stand up to Xanxus if he ever becomes a threat again. For Xanxus, he could start to see Tsuna as a more interesting opponent rather than just a nuisance. He might become more observant of Tsuna's growth, which could lead to a more complex relationship between them.
Obviously, the students of Class 1 - A would be main characters. There are so many of them with diverse personalities like Midoriya, Bakugo, and Uraraka. They would bring their own reactions and interactions to the story.