Nan's father died a month before she was born, leaving the girl's birth mother (Rebecca Adams) in a rough financial situation. Nan was born as the younger twin to a boy named Montgomery "Monty" Adams. Nan's true mother couldn't afford to raise both children, so Nan was raised by her uncle, Leo, in Pennsylvania. Meanwhile, Monty was raised in California. Unfortunately, Nan's mother-figure (her biological aunt, Susan Matz) died when Nan was two years old.
After returning from a music camp, Nan is reunited with Monty, who has been left to live with Leo and Nan for a year. Monty thinks the girl is his cousin, but he finds out the truth during his stay, and Nan is informed later. Monty starts seeing an evil version of himself running around. As the twins learn, Uncle Leo created a clone of himself that turned out to be evil. This clone then created evil clones of Monty.
Nan and Leo mistake Monty for a clone, and he is hauled away. He escapes and returns to Nan. He tries to prove his authenticity, but he fails and is hauled away again to South America with other clones.
Nan is talented, as she's good at sports and music. She isn't too competitive and gets along well with Monty.
Nan is tall and thin, like Monty. Her hair is a slightly lighter shade of red than his. She has bangs and green eyes.
- Nan's last name is never explicitly stated in the book, but it can be assumed that her last name is Matz — not Adams. Otherwise, she would've known Dr. Leo Matz wasn't her real father.
- The last line of the book is directed at Nan and comes from one of Monty's evil clones, who says, "I'm your evil twin!" Technically, this makes Nan one of the titular characters. She is the "Your" referenced in the title, not Monty.