Character Analysis
Different from the Get Go
From her first interaction with Katsa and Po, we sense that Bitterblue is a somewhat extraordinary ten-year-old. After hiding in the woods from her murderous father for hours on end, she is informed that her mother is dead. After relaying the information to Bitterblue, Katsa "waited for sobbing, screams. But instead there was a pause, and then the voice came again. […] 'The king killed her?'" (24.35-26).
She doesn't say much else for a while, except to let them know she feels warmer and that she saw them fight off about fifteen of Leck's soldiers with ease. Then she informs them, "'You'll have to kill the king […] if you ever want him to stop chasing us' (24.68) with a "calmness in her manner of speaking" (24.69) that Katsa finds remarkable.
Stronger than a Lying Leck
And Bitterblue is remarkable. Like Katsa she has been abused by a family member, and also like Katsa she found the power to break free from the tyrannical hold he had on her. No, Bitterblue's break from Leck isn't quite as clean—or as effective—as Katsa's break from Randa, but she does what she can with the tools she has at her disposal, and she accomplishes something quite amazing: she develops partial immunity to Leck's Grace, something not even Katsa can do.
Remember, even as Katsa hurls her dagger at Leck, she is still under his spell, but Bitterblue manages to keep her head. She's not confused for one moment about the fact that Leck is a very bad man. She's just really scared. Once Leck is dead, however, we see Bitterblue's strength and intelligence unfold in force, and just as Katsa once protected her, she steps forward to protect Katsa.
Bitterblue Takes Charge
"'You will not hurt her'" (35.50), Bitterblue tells Po's father and brothers after Katsa kills Leck. And when King Ror suggests that Princess Bitterblue is not well, she counters, "'I'm perfectly well now that he's dead,' […] her voice growing stronger and her hand steadying. "And I'm not a princess. I'm the Queen of Monsea. Katsa's punishment is my responsibility, and I say she did right, and you will not hurt her'" (35.52).
So one reason that Bitterblue is an important character is that she too undergoes a transformation when she is freed from a tyrannical relationship, mirroring Katsa's journey (more or less) and suggesting that Katsa's self-defense classes for young ladies in the seven kingdoms are sorely needed.
Bringing Out Katsa's Softer Side
And Bitterblue also helps us see that Katsa is capable of placing someone else's needs above her own. Without Bitterblue, it would be all too easy for some folks to characterize Katsa as selfish. She is, after all, a woman who doesn't want to get married or have children, and sadly, women who don't take on those roles are sometimes viewed as cold or self-centered. (Sorry, gals.)
Yet when Katsa flees toward Grella's Pass with Bitterblue, she makes two big sacrifices. First she leaves Po behind, which rips her heart into tiny pieces; and second, by safeguarding Bitterblue, she puts herself in direct opposition to Leck, the one man she actually seems powerless to defeat. As Katsa and Bitterblue develop a kind of big sister-little sister dynamic, we see with certainty that Katsa doesn't have to want children in order to be a loving and committed caregiver.