Was Kristin Lavransdattar a real person?

Kristin Lavransdatter is a series of historical novels written by Sigrid Undset, consisting of three novels: The Wreath (Kransen), The Wife (Husfrue), and The Cross (Korset). These novels trace the life of the titular Kristin, who grows up in the Gudbrand Valley in Norway in the 14th century. The Wreath introduces Kirstin as the product of a religious, loving family—the daughter of a wealthy farmer, Lavrans, and a depressed mother, Ragnfrid. She is sent to a nunnery in Oslo after a man attempts to rape her, and she eventually falls in love with and marries Erlend Nikulaussøn. 
While Kristin's tale is a compelling one—and is actually the reason for Undset being named as the recipient of the 1928 Nobel Prize in Literature—the character of Kristin is just that: a fictional character. She was not a real person living in the Middle Ages.