McGonagall could be very maternal in her reaction. Despite her tough exterior, she has a soft spot for her students, especially Harry. She might take Harry aside to a quiet corner of the castle, sit him down, and put a comforting hand on his shoulder as she talks to him. She could offer words of encouragement and let Harry know that he's not alone in his struggle.
She might react with her usual no - nonsense attitude. McGonagall is known for being strict but fair. So she would first listen carefully to Harry's problem, her eyes fixed on him, and then start to analyze the situation. She may not show much emotion on the surface, but deep down, she's concerned for Harry's well - being.
She might be initially cautious. Given that becoming an Animagus is a complex and dangerous process, she would likely want to ensure Harry fully understands the implications. She could start by asking him why he wants to become one.
She might immediately comfort him. McGonagall is a strict but kind teacher. She could put a hand on his shoulder and say soothing words like 'There, there, Mr. Potter. It will be all right.'
McGonagall might be initially shocked. She's a strict teacher and not used to such an informal and deeply emotional form of address. But then she could feel a sense of warmth and responsibility towards Harry. She might start to show more care in her actions, like making sure he eats well in the Great Hall or checking on his studies more closely.
She could also provide Harry with some protection. In fanfiction, she might be even more proactive in keeping Harry safe from the dark forces. For example, she could give him a magical charm or a secret spell that only she knows to fend off enemies. Or she could arrange for some trusted allies to keep an eye on Harry when she can't be around.
Harry might be initially shocked and confused. He has always seen McGonagall as a reliable figure in his life at Hogwarts, so her abandonment would be a huge blow to him.
She might use her magic skills to shield Harry from physical harm. For example, she could cast protective spells like a strong shield charm when Harry is in danger from dark wizards. Also, she may use her authority in Hogwarts to keep him away from those who wish to harm him, like making sure he is not alone in dangerous areas of the school.
The main plot could be Harry's journey to become an Animagus with McGonagall's guidance. He might face various challenges like mastering difficult Transfiguration spells and controlling his emotions during the process.
McGonagall often serves as a mother - like figure in fanfiction. She might comfort Harry when he's facing difficulties with Voldemort or other challenges at Hogwarts. Her influence is seen in how Harry approaches problems. He learns from her to think calmly and use his wits, rather than just relying on his Gryffindor bravado. This helps him make better decisions and grow into a more well - rounded wizard.
McGonagall might notice Harry's distrust and feel hurt or frustrated. She may try to figure out what went wrong but find it difficult as Harry is not forthcoming. Their interactions become less warm and more formal. For example, in class, Harry may be less likely to ask for help from McGonagall, and McGonagall may be more strict with Harry as she doesn't understand his change in attitude.
In most proper Harry Potter fanfiction, McGonagall is a strict but fair character. If Harry were to break school rules in a fanfic, she might punish him as she does in the original books. However, it depends on the plot and the writer's interpretation of her character.