Mistle is central to the “The Witcher” book series. Her storyline closely resembles Ciri’s. As with her Netflix counterpart she’s a member the Rats as well as a thief. It’s a tragic story, she had to flee her hometown in order to escape violent suppression of a political revolution, but was sexually assaulted by male captors. She and several other women who had been on her perilous trip helped to establish the Rats.
Although the relationship between Ciri and Mistle, in the book series, is a romantic one (afterall the latter stole from the first), the dynamics are currently in flux. Ciri and Mistle form a strong bond in their time together, which is an important step in her emotional development. It’s unclear if all of this will be replicated on the Netflix series, but based on their initial interactions it appears that Mistle Ciri are going to find themselves intertwined at some point.