Peter might get a bit flustered at first. He's young and looks up to the Avengers, so being made fun of could make him blush or stammer. But then he usually bounces back with a witty comeback of his own. For example, he might say something like 'Well, at least I don't have an old - fashioned shield' to Steve if Steve teases him about his suit.
Well, sometimes in fanfic the Avengers make fun of Peter's suit. They could say it looks like a pajama or something. Maybe Tony Stark, in a teasing way, comments on how Peter's web - shooters are a bit too 'homemade' compared to his high - tech Iron Man suits. Another instance could be that the Avengers mock Peter for his clumsy moments when he first joins them, like tripping over his own feet while trying to show off his acrobatic skills.
It could also be due to jealousy. Peter might be getting too much attention from the public or other superheroes. His youth and new - found popularity could make the more established Avengers feel overshadowed, leading to feelings of resentment towards him.
It could be that Peter's immaturity showed in a crucial situation. He might have been over - confident and tried something really risky without thinking it through. The Avengers, being more experienced, would be frustrated with his recklessness and thus yell at him. They want him to learn and grow, but his actions might have been too much to tolerate at that moment.
He might also get angry. Peter has a strong sense of justice. If he feels that the Avengers are being unfair, he could stand up to them. He'll use his quick wit and his powers to show that he's not going to be pushed around easily.
One possible reason could be a misunderstanding. Maybe Peter accidentally gets involved in something that looks bad to the Avengers, like being in the wrong place at the wrong time during a security breach. So they misjudge him and are mean as a result.
They might notice signs like Peter being overly withdrawn, flinching at sudden movements, or having unexplained injuries. Maybe they see him constantly on edge around certain people.
Peter might be very defensive at first. He could argue back, trying to explain his actions. After all, he is a teenager with his own ideas and he may think that the Avengers don't fully understand his perspective.
In some fanfics, Peter might faint due to overexertion. He's often depicted as being involved in intense battles or using his powers to the extreme. For example, if he's been fighting a powerful villain for a long time without a break, his body could just give out and he faints.
Obviously, Peter Parker is one of the main characters. And then, the Avengers members like Tony Stark, Steve Rogers, Natasha Romanoff, etc. are also main characters as they are the ones adopting Peter. They play important roles in Peter's new life and in the overall story.