She only appears in the Death Minigame, in Sister Location, and you can hear her voice at the start of every night(and in the Fake Ending). The only time we see her.
Before the events of Sister Location
Chica's Party World/Death
During her birthday party at Circus Baby's Pizza World, she goes against her father's wishes and approaches Circus Baby, who ends up luring her closer using ice cream, and killing her.
In the Minigame, it shows Baby swallowing her with what seems to be a giant claw protruding from her chest, though it could be anything. Ennard (through the voice of Circus Baby) says that, because the kids were screaming and cheering so loudly for the other animatronics, no one could hear her dying.
Sister Location Dialogue
Night One: "Daddy, why won't you let me play with her?"
Night Two: "Daddy, you let the other children go see her! Why won't you let me go?"
Night Three: "Daddy, just once let me go play with her! She's so pretty and shiny! Didn't you make her just for me?"
Night Four: "Daddy she can make balloons! Have you seen her make balloons? Oh, daddy let me go to her!"
Night Five: "Daddy isn't watching."
Ennard: "Hello?" "Hellooo?" "Hello...?" "We need you so we can leave!" "I don't understand!" "I know it was an accident!" "Everything is okay. I'm still here." "You have to let me inside the room!" "Isn't this why you came?"
It's revealed that the pigtail girl and Elizabeth are 2 different people,confirming that pigtail is not related to the Aftons.