Bakugo is a likely candidate. Despite their rocky relationship in the beginning, Bakugo has come to respect Izuku's growth and strength. In a fight against All for One, Bakugo's powerful quirk and combat skills would be a great asset.
Ochaco Uraraka could be there to support Izuku. Her zero - gravity quirk could be used in creative ways to disrupt All for One's attacks or movements. Also, she has a strong bond with Izuku and would be motivated to help him in such a crucial battle.
All Might would probably be in it. Since he's Izuku's mentor, he could play a role in helping Izuku understand and control his time - controlling power. Maybe he has some past experience related to time powers that he shares with Izuku.
A third plotline could be that Izuku has to fight All for One not just physically, but also in a battle of wits. All for One is a very cunning adversary, and Izuku would need to outsmart him. This could involve Izuku using misdirection, setting traps, and predicting All for One's moves in order to gain the upper hand.
Obviously, Bakugo would be one of the most affected. His relationship with Izuku has always been complex, and Izuku becoming a girl would surely change the dynamic between them. He might have to re - evaluate his feelings of rivalry and superiority. All Might would also be affected. As Izuku's mentor, he would need to adjust his training methods and support system to fit Izuku's new situation as a girl.
Izuku is surely one of the main characters since it's in his name. His friends from the original series like Ochaco Uraraka and Katsuki Bakugo are also likely to be there. They play important roles in Izuku's life and would fit well in a story that focuses on Izuku's 'fist of love'.
His friends from U.A. High School such as Bakugo, Ochaco, and Iida would also likely be main characters. Since Izuku is scaring everyone, they would be the ones experiencing the fear and trying to figure out what has happened to him. Their reactions and attempts to deal with the situation would be important parts of the story.
Bakugo is a very likely opponent. Their rivalry is well - known, and in the sports festival setting, it would be an intense battle with Izuku using his full cowl against Bakugo's powerful explosions.
One possible supporting character could be Bakugo. He might be initially reluctant to help Izuku but then realizes the importance of the fight against All for One and joins in. His explosive powers could be a great addition to the battle.
In Kidnaped Izuku Fanfiction, likely antagonists could be rival villains who see Izuku as a threat to their plans. For example, those who want to stop him from becoming a great hero. It could also be villains who are after his unique quirk. Maybe they want to study it or use it for their own evil purposes. Another possibility is that it could be a group with a vendetta against the hero school he attends, and they target Izuku to send a message.
In the fan fiction, Izuku might show respect for women by promoting their leadership. He could step aside and let female characters take the lead in certain situations when he realizes they are more capable or have a better understanding of the situation at hand. This shows that he values their skills and judgment.
There could be some original characters created by the fanfiction writer. For example, a mysterious mentor figure who has been secretly training Izuku in a special technique that he can use against All for One. This mentor could have their own backstory and motives that are intertwined with the main plot of the fight.