If we're talking about the core characters in Kristin Lavransdatter, you really have to start with Kristin herself. The entire trilogy follows her from a young girl through her entire life, so her journey is the absolute centerpiece. Her father, Lavrans Bjørgulfson, is massive in the first book, 'The Wreath'—his relationship with Kristin, his quiet honor, and the weight of his disappointment shape her forever. Ragnfrid, her mother, is fascinating in a quieter, more sorrowful way; you see how her own past mistakes affect how she mothers Kristin.
Then there's Erlend Nikulausson, the knight Kristin falls for. He's charming and reckless, and their passionate, tumultuous marriage drives most of the plot tension after they wed. I've seen readers get incredibly frustrated with him—he's not a villain, but he's perpetually irresponsible, which clashes with Kristin's more conscientious nature after she matures. Their sons are important, especially the eldest, Nikulaus, who becomes a monk, and Ivar and Skule, the twins who take after their father's adventurous spirit. Simon Andresson, the steady man Kristin was first betrothed to, is a heartbreaking figure of 'what could have been' for a lot of readers; his loyalty and unrequited love add a deep layer of tragedy.
Honestly, the secondary characters like Lady Aashild, Erlend's aunt who gives Kristin her initial push toward Erlend, and Brother Edvin, the wandering friar who gives her spiritual guidance, are crucial too. They represent the different forces—worldly wisdom versus faith—that pull at Kristin throughout her life. The cast feels less like a list of characters and more like a whole community you watch grow and change over decades.