Her stern demeanour hid a warm personality and fierce protectiveness. Here’s the proof…
McGonagall with hand on Harry's Shoulder from the Prisoner of Azakaban.

A sharp tongue, a severe bun and narrowed eyes through glinting spectacles. Professor McGonagall was a no-nonsense woman who didn’t suffer fools at all, let alone gladly. However, her stern nature actually hid a warm personality, not to mention fierce protectiveness. As we learnt, she truly cared for the kids at Hogwarts – even if at times you might not have seen it.

When she couldn’t stand the idea of leaving baby Harry on a doorstep

Professor McGonagall didn’t often let her emotions show, especially in professional contexts with her colleagues. But when it came to a certain moment at Privet Drive, McGonagall got misty eyed, and understandably so.

For a full minute, the three of them stood and looked at the little bundle; Hagrid’s shoulders shook, Professor McGonagall blinked furiously and the twinkling light that usually shone from Dumbledore’s eyes seemed to have gone out.
Harry Potter and the Philosopher’s Stone

Dumbledore and McGonagall leave baby Harry at Privet Drive

When Harry and Ron told Professor McGonagall they wanted to visit Hermione in the hospital wing (even though they were lying)

Aside from the fact that Harry and Ron were manipulating Professor McGonagall’s little-seen emotions to get their own way, this scene was very touching. Harry and Ron probably weren’t expecting their plan to work so well.

Professor McGonagall was still staring at him, and for a moment, Harry thought she was going to explode, but when she spoke, it was in a strangely croaky voice. ‘Of course,’ she said, and Harry, amazed, saw a tear glistening in her beady eye.
Harry Potter and the Chamber of Secrets

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When McGonagall was on Team Neville

McGonagall encouraged Neville to follow his own path, even if it wasn’t what what was expected of him.

‘Humph,’ snorted Professor McGonagall. ‘It’s high time your grandmother learned to be proud of the grandson she’s got, rather than the one she thinks she ought to have – particularly after what happened at the Ministry.’
Harry Potter and the Half-Blood Prince

Neville with his Mimbulus Mimbletonia in a Herbology class

When McGonagall supported Harry instead of Professor Umbridge

Well, obviously she supported Harry over Umbridge. But for a woman who took her job – not to mention rules – very seriously, this was a big step.

‘Is it true that you shouted at Professor Umbridge?’
‘Yes,’ said Harry.
‘You called her a liar?’
‘Yes.’
‘You told her He Who Must Not Be Named is back?’
‘Yes.’
Professor McGonagall sat down behind her desk, frowning at Harry. Then she said, ‘Have a biscuit, Potter.’
‘Have – what?’
‘Have a biscuit,’ she repeated impatiently, indicating a tartan tin lying on top of one of the piles of papers on her desk. ‘And sit down.’
Harry Potter and the Order of the Phoenix

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When McGonagall expressed the emotions we were all feeling

And sent shivers down our spines.

He waited. Any moment, the people for whom he had tried to die would see him, lying apparently dead, in Hagrid’s arms.
‘NO!’
The scream was the more terrible because he had never expected or dreamed that Professor McGonagall could make such a sound.
Harry Potter and the Deathly Hallows

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When McGonagall believed in her students. All of her students

At the core of her personality – before being a member of the Order of the Phoenix, or being a total bad-ass – McGonagall was a teacher. She was there to nurture and grow talent in her students. And that involved believing in them – which she did unfailingly. Just ignore the brisk tone in her voice.

‘You cannot pass an O.W.L.,’ said Professor McGonagall grimly, ‘without serious application, practice and study. I see no reason why everybody in this class should not achieve an O.W.L. in Transfiguration as long as they put in the work.’ Neville made a sad little disbelieving noise. ‘Yes, you too, Longbottom,’ said Professor McGonagall. ‘There’s nothing wrong with your work except lack of confidence.’
Harry Potter and the Order of the Phoenix

An illustration of McGonagall's robes
Harry Potter to Fantastic Beasts