Well, Izuku is a given. But we can't forget about the villains too. In this fanfiction, villains like Tomura Shigaraki could be involved in Izuku's spiral. Maybe Shigaraki has a plan that somehow involves Izuku's new 'spiral' situation. And characters like Mei Hatsume from the support department could also be part of it, perhaps creating some new gadgets for Izuku to deal with the challenges related to the spiral.
Izuku is the main focus, but Katsuki Bakugo could also be a major character. Given their complex relationship in the original series, Bakugo could either be a source of conflict or an unexpected ally in Izuku's spiral. Additionally, the teachers at U.A., like Aizawa, might be involved. They could be the ones trying to figure out what's going on with Izuku and help him get out of the spiral if it's a negative one.
Obviously, Izuku Midoriya will be the central character. His friends from U.A. High like Ochaco Uraraka and Tenya Iida are likely to be involved. They would be there to support or oppose Izuku depending on the nature of the spiral he's in. Also, All Might might play a role as Izuku's mentor, guiding him through whatever challenges the spiral presents.
I think the students like Kaminari and Sero would be involved. Kaminari with his goofy personality might initiate some pranks, and Sero could use his tape quirk to set up some elaborate prank setups. Also, the teachers like Aizawa could be involved. He might play a prank on the students to teach them a lesson in a rather unorthodox way. Another character could be Todoroki. His rather stoic nature could be a great contrast to the chaos of a prank.
Uraraka is another character who could be involved. She has a close relationship with Izuku in the original series, and in this fanfic, she might be there to support Izuku in his new journey where he doesn't face the same physical limitations. Her own growth as a hero could be intertwined with Izuku's newfound abilities and lack of crippling injuries.
One possible plot idea could be Izuku developing a new and unique quirk spiral ability that sets him apart from his peers. Maybe it starts as an unexpected mutation of his existing power. Another idea could be Izuku getting caught in a time - spiral within the My Hero Academia universe, where he has to relive certain events to change the outcome for the better. And a third could be Izuku's spiral being related to a spiritual or mental journey, where he discovers hidden aspects of himself and the hero world.
The main characters would obviously be Izuku Midoriya and Rei Todoroki. Izuku is the protagonist of the main series, a quirkless boy who dreams of becoming a hero. Rei is Shoto Todoroki's mother, who has her own complex backstory and powers. In a fanfiction, they would be the central focus, with their interactions and character development driving the story forward.
Izuku Midoriya is a likely character. He has a strong sense of justice and is always willing to put others before himself. His determination to be a hero makes him a prime candidate for self - sacrifice. Bakugo Katsuki could also be involved. Despite his tough exterior, he has a good heart and would sacrifice for his friends and to prove himself as a true hero.
Definitely Midoriya. He's such a central figure that it's easy to imagine how hypnosis could play into his story. He could be hypnotized to doubt his own abilities or, on the flip side, be hypnotized to unlock new levels of his quirk. Ochaco Uraraka is another character who could be involved. Her zero - gravity quirk could be manipulated through hypnosis. For example, she could be hypnotized to lose control of her quirk in a dangerous situation or be made to use it in a more powerful and unexpected way.
Definitely Izuku Midoriya. He's the main protagonist in the original series, so he would probably play a crucial role in any 'Days of Future Past' type of story. He could be the one leading the charge to change the future.
In a darker take, even some of the younger students at U.A. High could die. This would be a very emotional and impactful event. It would show the harsh reality of the hero world and how not everyone is guaranteed to survive, no matter how much potential they have.
Ochaco Uraraka is a common character in the harem. Her kind and friendly nature makes her a likely candidate. Momo Yaoyorozu is also often included. Her intelligence and elegance add an interesting dynamic to the harem. And Tsuyu Asui could be part of it too, with her unique personality.
Definitely include All Might. He could be like a guiding figure for Izuku during the summer. He might give Izuku special summer training or pass on some important wisdom. Another character would be Iida. His strict sense of order and strong leadership skills could be useful in a summer group activity with Izuku. For example, they could be part of a team that has to organize a big hero - themed event. Also, consider adding Mei Hatsume. Her crazy inventions could play a role in Izuku's summer. She could create some new gear for him to use during his summer training or adventures.