Miri is a fourteen-year-old girl who's small for her age—so small, in fact, that she suspects it's the reason her father won't let her work in the quarries (much to her dismay):
Miri had been tiny from birth and at age fourteen was smaller than girls years younger. There was a saying in the village that when something was thought to be useless it was "skinnier than a lowlander's arm." Whenever Miri heard it she wanted to dig a hole in the rocks and crawl deep and out of sight. (1.30)
But despite her size, Miri is surprisingly strong-willed and able to face off against much larger opponents. She's a quick thinker who is able to stand up to Tutor Olana and use her diplomacy skills to escape punishment and negotiate terms for the girls' education, even though Tutor Olana is quite formidable (and prone to stuffing girls in the closet like some G-rated version of Bluebeard). Even when she's faced with a real life bandit, Miri stands up for the other girls and plots their escape, taking the lead with gusto:
She saw their faces turn to that dull point of light and flicker with apprehension and fear. Miri could not risk having anyone stay behind. With her eyebrows raised as if she posed a question, Miri pointed at each girl and waited until she nodded in agreement. (22.18)
In the end, Miri is able to use her smarts and fearlessness to overcome Dan—even if he may be much bigger and meaner than her. Miri may look like a little girl, but you should never underestimate her.
In addition to being a fearless little warrior, Miri is also a force to be reckoned with inside the classroom, so even though she doesn't like Tutor Olana's attitude, Miri soon settles into the rhythm of lessons and ends up being a star pupil. Miri isn't just taking the lessons seriously because she wants to become academy princess (like Katar), either—Miri is interested in learning because it opens up new worlds and possibilities to her.
Eventually Miri finds that all she wants to do is continue reading, learning, and opening up this same world to others. Instead of opting to go to Asland and be a part of the king's court at the end, Miri decides that her place is on Mount Eskel, teaching her fellow villagers. She tells Peder that she'd rather help out the community:
[…] And I want to open an academy in the village where anyone can come learn. Esa is going to help me. (25.150)
After all, Miri understands the importance of education and what it can do for people, and she wants to continue sharing that knowledge with the people closest to her.
Miri may be competitive (after all, she's gunning for the title of academy princess), but she definitely wants to keep things fair. Even though there's no lost love between her and Katar, she still helps Katar out during the final exam, since all of the other girls helped each other through quarry-speech too. Miri doesn't want to, exactly, but she knows it's the right thing to do:
She did not want to give Katar anything, but her sense of justice would not allow her to help every girl but one. Miri glared at Katar, slapped her hand on the grass, and sang mutely of Olana's introductory lecture on Poise. (16.50)
Miri is a girl of strong principles. And it's that same adherence to fairness that makes her want to be a teacher instead of a princess in the end. After all, she had this amazing opportunity to learn how to read and about all sorts of subjects through the princess academy, but what about all the other people in the village? How is it fair that they don't get to learn too? That's why Miri sets out to make sure that everyone can be educated if they want to be:
And Marda was smart. It was injustice and not luck at all that made Marda three months too old to attend the academy. And not just Marda—what about the younger girls? And all the boys? (12.65)
Maybe that kind of fairness and sense of justice would have served Miri well as a court delegate, but she doesn't want any of that. She knows that she's just as important to the people of Mount Eskel, and that's where she belongs.