Maybe the female reader is a scholar who is interested in the same ancient mysteries as Kassandra. They meet while researching in an old library. Together, they embark on a journey to uncover a long - lost artifact, facing various challenges along the way like traps and rival treasure hunters. Their relationship develops as they rely on each other's skills and knowledge.
Here's a different plot. The female reader is a young warrior who idolizes Kassandra. She manages to join Kassandra on one of her quests. At first, Kassandra is a bit skeptical about having the reader along, but as they face difficulties together, Kassandra sees the reader's potential and determination. They become friends and learn from each other, with the reader learning combat skills from Kassandra and Kassandra learning about the reader's unique perspective on life.
One idea could be that the female reader is a healer in Kassandra's village. Kassandra gets injured during a battle and the reader takes care of her. They start to form a bond as the reader teaches Kassandra about different herbs and their healing properties, and Kassandra shares her tales of adventure.