Sonia is the one of safehouse keepers of the children about to be sent to the Graveyard. She keeps them in the basement of her antique store in Ohi and lets them wait for the "ice cream man" to come. Before they leave, though, she makes them write a final letter addressed to a loved one which she promises to either deliver herself if they have been unwound, or return to them if they survive past the age 18/17.
It is then revealed in UnWholly that Sonia is the wife of the scientist, Janson Rheinschild, and together, they introduced unwinding to the world, not knowing that it will be used for the reasons it is being used in the current time.