Nixie Knox is a girl in the book Dr. Seuss's ABC. She is only seen in a portrait.
Marian Hailey Moss the narrator explains that the letter "X" can be very useful in her name.
In the Beginner Book version of this book, Her picture frame falls down after being seen for a brief moment.
In the Living Books version she comes to life letting everyone know who she is yet she proclaims she is boxed in her frame and can't seem to get out.
If you flip her picture frame, you will see a Zed who proclaims this isn't where he thought he would be and he notices a fox, then he leave and the frame of Nixie is shown again.
Her favorite instrument is the saxophone.