Britta is the only girl at the princess academy who originally comes from the lowlands, and because of this distinction she's pretty much ostracized by the other girls. The other girls assume that since she's a lowlander she must think that she's above them all (all lowlanders they've met have been snooty):
Miri coughed. "He does? I don't think she is. I mean, she acts like she's too good to talk to anyone."
"All lowlanders think they're above us," said Frid. (3.69-70)
The truth though is, though, that Britta is simply lonely. She's arrived on Mount Eskel by herself and spent a whole season working in the quarries where no one talks to her, since everyone else communicates via quarry-speech. When Miri starts talking to her, Britta is incredibly relieved.
Britta also displays some serious awkwardness when it comes to Prince Steffan. She's so embarrassed by the prospect of him rejecting her that she gets sick during the ball and can't come out to meet him. When he comes a second time, she's equally frantic and afraid to see him—even though she's known him for forever:
Britta put a hand to her stomach and groaned. "I can't do it, Miri. I think I might actually die." (25.52)
But in the end, with the help of Miri, Britta gets over her social anxiety and her irrational fear of seeing the prince again. She just needed someone by her side to help her through the process of making new friends and fighting for love.
When Miri becomes friends with Britta, she recognizes that the other girl is trustworthy and can keep a secret. Because of this, Miri chooses to tell Britta about what she's learned about quarry-speech and Commerce before anyone else:
Would Britta believe her? Or would she laugh? Miri thought of Doter's saying, Never hesitate if you know it's right. After months of ignoring Britta just for being a lowlander, at least she deserved Miri's trust. (9.6)
But Britta isn't just good at keeping other people's secrets—she has a few of her own that she keeps to herself throughout the whole book, not even cracking when Tutor Olana asks her to lead the class or when the girls start speculating about what Prince Steffan must be like. You see, Britta's not actually an orphan; her parents are nobles who sent her up to Mount Eskel in order to be considered as the prince's bride. She doesn't breathe a word of this to anyone until right before Prince Steffan returns to select his bride:
Britta whimpered and put her face in her hands. Her voice came out muffled. "I hate this, Miri. I should've told you before, but whenever I thought about it I felt so ill and embarrassed and horrified and—" (25.30)
Britta isn't being true to herself when she keeps all those personal secrets, but when she sets everything out on the table, it all turns out just fine. She doesn't lose Miri's friendship or respect like she'd feared, and Prince Steffan is delighted to see her; after all, he was in love with her too.