Who is Sonia?
In the book Unwind, Sonia is a fairly minor character. She is important for sure, but she gets very little page time. Her background is explored in much greater detail in the sequel book UnWholly.
In Unwind, Sonia operates an underground railroad of sorts. Instead of secretly spiriting away slaves, Sonia helps runaway Unwinds make their way to the Graveyard. She does this by housing the Unwinds in her basement and arranging safe transport to Arizona. While staying in her basement, Sonia requires each Unwind to write a letter to a loved one. She keeps the letter until the Unwind turns 18. At that point, the child is no longer in danger of being unwound. The child can then return for the letter and personally mail it out or destroy it. If the child is unwound or does not return for the letter, Sonia mails it out herself. Connor and Risa both benefit from her work and her protection in the book Unwind. Unfortunately, so does Roland.