Well, obviously Izuku is one of the main characters, even when he's turned into a cat. His friends like Bakugo and Ochaco are also likely to be main characters. They would be the ones trying to figure out what happened to Izuku and how to turn him back. And maybe All Might could also be a main character, if he gets involved in the whole situation to help his young protege.
A more emotional plot could be Izuku as a cat being found by a villain who initially wants to use him against the heroes. But over time, the villain starts to develop a strange bond with the cat - Izuku. This then makes the villain question his own motives and actions.
He would likely be more confident. Having the powers of the Human Torch gives him a new level of strength. So he might stand up straighter and be more assertive in his actions.
In many fanfictions, Izuku adapts by relying on his heroic spirit. He sees the similarities between his world and the Spider - Verse in terms of helping people. So he starts to work with the local heroes, like Peter Parker or Gwen Stacy. He learns from them about the different threats in the Spider - Verse and how to handle them. He also tries to incorporate his own fighting style from My Hero Academia with his new spider - based powers. For instance, he might combine his One For All power with his new spider - strength for more powerful attacks. This process of adaptation is really exciting to read about as it shows Izuku's growth in a new and different environment.
His friends like Ochaco could help. She might use her zero - gravity ability to assist in handling heavy parts during the construction. Also, Iida could be there to keep things organized and on schedule with his strict sense of order.
Bakugo is often a key character. In many fanfictions, Bakugo's reaction to Izuku's evolving quirk can range from envy to begrudging respect. Their relationship is always a central focus, with Bakugo either trying to outdo Izuku or having to come to terms with Izuku's newfound power.
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 often appears. Since he was Izuku's mentor, in time travel scenarios, he might be the one Izuku turns to for advice or assistance. Another common character is Bakugo. Their complex relationship can be explored further in time travel stories, like if Izuku goes back in time, he might try to change how Bakugo treated him in the past.
There could be his friends like Ochaco Uraraka. She might be in a position where she's trying to understand Izuku's situation and help him. Maybe she tries to convince the people forcing Izuku to give him some space.