The Snake-Charmer Girl is a female peculiar with the peculiarity of a reign over snakes.
The Snake-Charmer Girl first appeared in the sideshow. Emma knew she was a peculiar because only peculiars could have control over snakes. She later appeared in the congress of human oddities and it is confirmed she is part of Sergei's army.
When Jacob was calling a roll call, it was confirmed she was also captured and having her soul drained. She was probably rescued after the events of Library of Souls.