Sophie Piper is the sister of Axel von Fersen.
In 1791, after the attempted escape of the royal family with Axel, Sophie got ahold of a letter sent by Magnus Bane to her brother. She wrote back to him and told him of Axel's status: that though he was fine, he was in Vienna, pleading their case to the Emperor. Concerned for Axel's safety, she then told Magnus of Axel's plan to return to Paris for the family, at the risk of his own life, and asked Magnus to try to talk Axel out of it—a request Magnus would choose to ignore.
- Like Axel von Fersen, Sophie is also based on a real person, though her surname is never stated in the Bane Chronicles story she was featured in.
- The Runaway Queen (first and only mention)