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.
Perhaps when Harry is wrongly accused of something, and McGonagall uses her influence and knowledge to prove his innocence. She stands up for him in front of the whole school or the wizarding authorities, which is a very significant moment of her protecting Harry.
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.'
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.
One possible reason could be that she is under some sort of dark magic influence. Maybe a spell has been cast on her that makes her act out of character and abandon Harry.
One common plotline is McGonagall protecting Harry from the Ministry of Magic when they wrongly suspect him of something. For instance, if the Ministry believes Harry is using Dark Magic, McGonagall stands up for him, using her influence and knowledge of Harry's character to prove his innocence.
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.
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.
It could also be due to some actions of McGonagall that seemed suspicious to Harry. Perhaps she made a decision regarding Hogwarts or Harry's safety that Harry didn't fully understand at the time. In the complex world of the wizarding school, decisions are often made with many factors in mind, but Harry might not be aware of all of them, leading to his distrust.
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.