Have you ever heard of Phineas Gage? I'm gonna keep this simple, (Wikipedia has the gross details) but a man in the 1800s suffers a head injury he probably should have died from, and drastically changed as person afterwards to the point where his friends literally didn't see him as the same person anymore.
This can happen naturally too. Sometimes friends grow apart. Maybe you realized that you can't tolerate their quiet Republican bullshit anymore as a budding Socialist. So you drift. I wonder how it was for Gage's friends (Wikipedia doesn't say), did they just, one day, stop visiting?
But this can happen in fiction for fantastical reasons, no brain trauma required. And it's fun to look at. Stop at this point if you don't want to know anything at all about Monster Pulse or The Candle and The Flame. You can go read them and come back you know. Monster Pulse is free and online (feel free to @ me about MP, even if you just want a link), and the The Candle and the Flame is probably at your local library.
My favorite character in Monster Pulse is Violet, even though she isn't really important until Chapter 14, other than a brief thing before. And her emotional arc takes another 8 or so chapters to unfold, while other stuff is going on. You initially see her as this really “cool” girl, who seems very collected. She's very nice. She's helping a homeless boy her age do things he should be doing at his age, like enjoy a semblance of a school life, and then like I implied, she asks him out which of course makes her evil for a certain shipper type.
When it's time for Abel to go on his date with Violet, he meets her dad when picking her up, and learns she never tells him anything, that's normal right? Hahaha. Her father tries to tell Violet he loves her, but can't complete the sentence, and gives her money so she can enjoy her date. She understands. She tells Abel: “Money is often used when affection is hard to express.” twice, the first time about her father, the second to explain to explain to Abel it's okay that he has no money and she wants to pay for it. Her body language suggests embarrassment.
Eventually, it's revealed she has a Brain monster, Anima, and when Anima was created, all her emotions were amplified. So she has to maintain a cool facade because expressing slight nuances of emotion is very difficult, and if she lets people see the intense amount of emotion she feels, they freak out. Her father doesn't understand why she's changed so much, and it clearly distresses him that she's like this, but there's not much Violet can do.
There's a bunch of stuff I've taken out, but she's on better terms with her father now.
But this is something I can relate to. I don't know if it's an autism thing or what, but expressing shades of emotion is very hard. It's just much easier to try to suppress it all the time, especially if there's some sort of bad response when you express emotion. There's other reasons for that too, of course, but when you see a character that talks like a college professor, at like 15, and expresses emotions in degrees of extreme or none, you relate to them on an autism level.
The beginning of the book starts with an Ifrit, Ghazala, saving an infant, placing them in a nearby city, and performs a forbidden rite that transfers her Djinn Fire to the infant.
Nothing seems to come of it other than it healing the infant until partway through the book. Until then you've just seen this really passive girl, Fatima at the reigns. Then a tragic incident sparks her to change. It ignites her fire, and causes her to rename herself Fatima Ghazala (and she will demand to be referred to as such). Fatima Ghazala doesn't just sit back and accept things. When her sister says mean things to her, she responds assertively.
And her friends and family have to accept this new person. This literally is a different person, that happens to have the memories of the old person, and also has magical powers.
There's a part where she burns someone because they wouldn't stop harassing her, and she's: 1. Viewing the person the way you view a gnat (her words, not mine), 2. Annoyed she may have to face punishment for it, 3. Wondering what this would have been like if she had been fully human and couldn't, you know, set herself on fire.
For comparison Fatima dressed less femininely and more drab at the beginning of the story because she was afraid of harassment.
In a way, you get to understand Fatima better by seeing Fatima Ghazala's words and actions. If you've read Monthly Girls' Nozaki-kun, you know that one strip about dodging the spotlight? That's Fatima.
I won't say Fatima Ghazala eagerly accepts the “call to adventure” (using Monomyth for ease, not because I like it) because as the true protagonist that appears at roughly the 25% mark, she has to grow as a character. So there was some refusal.
The actual plot was about a war and a fighting royal family, but I was much more interested in exploring the acceptance of a radically changed person. (Don't take that as a slam against the book, I was interested in the plot plenty)
While Violet will always be the literal best, because even if Magnolia makes someone better, I don't think Violet can be replaced in my heart. But for comparison: Violet is a Secondary Character; I think Fatima Ghazala is what you get when you make the person with the story of sudden radical change your protagonist, which I found really interesting!
I wasn't the first to come up with the idea of equating Violet's change to brain trauma. I think I saw a commenter in that arc of the comment mention that, but the Phineas Gage comparison is my own.