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.
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.
Obviously, Izuku is the main character. He's the one who is mastering One For All. All Might also often appears as he was the previous user and is Izuku's mentor in many fanfictions. Bakugo can also be a significant character as he serves as both a rival and sometimes an unexpected ally to Izuku in his journey with One For All.
One interesting plot development could be Izuku unlocking a new power during the fight. This new power gives him an edge and surprises All for One.
Obviously, Izuku is a very important character as the main protagonist. The previous One For All users also play crucial roles. They could include All Might, who was the most recent user before Izuku. Each of the other past users, whose identities may not be fully known in the original story but could be explored in the fanfiction, are also important as they are the sources of the power's heritage.
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.
One potential side character could be the Thenardiers. They might try to take advantage of Cosette's pregnancy, thinking they can get some money or favors from Marius and Cosette. Madame Thenardier could be especially devious in her plans. Another side character could be a kind midwife in the neighborhood. She would help Cosette throughout her pregnancy, giving her advice on health and child - rearing. And of course, Gavroche, Marius's little friend, could be around, bringing some cheer and a sense of innocence to the story.
In a lot of 'naruto fights pain fanfiction', Hinata is a common side character. She might be in the background cheering Naruto on, or she could even jump in to save Naruto at a crucial moment. Shikamaru is another one. He could be using his intelligence to analyze Pain's moves and relay that information to Naruto during the fight.
There are several potential characters. Aizawa could be one. Godlike Izuku could show up in the past and Aizawa might be suspicious of his powers at first but then come to realize his good intentions. Gran Torino is also a possibility. Izuku could learn different things from him in the past. And then there are the other pro - heroes. They might see Izuku as a strange new force but also recognize his potential to do good. His interactions with them could change the course of hero history in this fanfiction.
Obviously, Izuku Midoriya is the main character as he is the one mastering One For All early. Katsuki Bakugo might also be prominently involved. Since he has a long - standing rivalry with Izuku, his reaction to Izuku getting such a powerful quirk early on could be a major part of the story. Maybe he feels jealous or challenged in a new way.
A likely main character could be a young ninja who discovers he has the 'One for All' power. He would be on a journey to understand and control it, while also dealing with the normal challenges of being a ninja in the Naruto world, like training and making friends.