Definitely Aunt May. She would be super worried about baby Peter and would do everything she could to take care of him. She might even call in some old friends or neighbors to help out.
Ned and MJ would also be important characters. Ned would be freaking out at first but then would quickly try to come up with plans to help Peter. MJ, on the other hand, would be very practical. She might start researching any possible ways to reverse the transformation. They would both be by Peter's side, whether it's playing with him as a baby or trying to solve the mystery of his transformation.
Ned could be another main character. He could come over to Peter's place to keep him company while he's in pain. They could end up playing video games or Ned could be trying to cheer Peter up by showing him some new Star Wars collectibles he got.
Perhaps his best friend. This friend could be the one who was with him right before the accident and blames himself for not stopping Peter from getting too close to the tracks. He would then be a key character in the story, trying to make amends in some way while also being there for Peter's family.
Mary Jane could be a great addition. She could be the one who helps Peter deal with the emotional turmoil of having Venom as part of him. Another good character is Ned. He could be the tech - support, trying to find ways to control or enhance Venom's powers. And of course, there's Doctor Octopus. He could be a great villain in the fanfiction, either trying to steal Venom's powers or team up with it against Peter.
One interesting plot could be that the other Avengers have to take care of baby Peter. They struggle with things like changing diapers and feeding him, all while trying to figure out how to turn him back. It would be really funny to see tough heroes like Thor or Iron Man being clueless about baby care.
Maybe it's his sister, Mary. Mary could have some idea about why Peter has locked himself away. She might be torn between respecting his privacy and wanting to help him. She could be bringing him food and leaving little notes outside his locked door, trying to communicate with him.
It could be a vengeful criminal that Spider - Man has put away in the past. This criminal has spent years in prison plotting his revenge and finally gets the chance when Peter is vulnerable. Maybe he manages to ambush Peter in an alleyway while he's on his way home from school or work.
His personality might become more in touch with his emotions. As a girl, Peter could be more empathetic and sensitive. He might also be more cautious in some situations, as society often treats girls differently. For example, he might not be as quick to jump into a fight without thinking about the consequences more carefully.
The main antagonist could be a scientist who wants to study the chibi. This scientist might be a bit of a madman, seeing the chibi as a unique specimen to experiment on and not caring about its well - being. He could try to capture the chibi and Peter has to stop him.
The AI itself could be a main character. It could have a name like Nova or something cool. And then there could be Peter's aunt May, who might be involved in the story when she discovers Peter's AI project and has some concerns about it at first but then comes to see its potential.
There could also be new characters created specifically for the fanfiction. Maybe a new love interest for one of the Peters, or a sidekick that is unique to this particular fan - made story. This new character could bring a fresh dynamic to the relationship between the two Peters.