Izuku would probably be terrified at first. After all, he's been kidnapped by villains. But then, his hero - like determination would kick in. He might start looking for ways to escape or at least resist their plans.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.
Eventually, he might come to accept it in a grudging way. If Izuku is wearing the clothes for a good reason, like in a team - up situation where it's part of a strategy, Bakugou might tolerate it. But he'll still probably make some snarky comments about how Izuku better not ruin his clothes.
All Might would probably be very supportive. He's always been about the spirit of heroism, not gender. He'd encourage Izuku (as a girl) to keep training hard. Bakugo might initially be shocked and then try to act all tough, but deep down he might be a bit confused about his feelings towards her. Ochaco would be excited to have another girl around and might become even closer friends with Izuku.
Some in 1A might be in awe. For example, if Izuku has a particularly powerful or unique quirk in fanfiction, characters like Mineta might be jealous. He often shows envy towards those with strong abilities.
The Afton Family story isn't real. It's a made-up concept that exists within certain fictional universes. These kinds of stories are created for entertainment and imaginative purposes only.
Besides William Afton, the 'william afton fnaf graphic novel' could feature a host of characters. The animatronics are likely to be major players. Freddy, with his leadership among the animatronics, might be important. Bonnie, the guitar - playing bunny, and Chica, the chicken. There could also be human characters such as the original founders of the Freddy Fazbear's franchise who might have had some connection with Afton. Additionally, any characters who have tried to solve the mysteries surrounding the spooky events at the pizza places could be part of the main cast.
Izuku might be shocked at first. After all, he has grown up without his father. But then he could be filled with a mix of emotions like joy, confusion, and anger all at once.