Ivy Smith is David and Eric's little sister. She was six years old. Her voice was changed to Ivy from Ella all because she cheated off a test. Later on, her voice was changed to Steven because she became a fan of Sesame Street and then it was changed to Shy Girl after getting her brother Eric arrested. She wanted her voice to be Ivy but since the doctor couldn't change her voice to Ivy, she stuck with Salli as her voice.
Salli and Ivy sound similar. When you make them say, "Waaaaaaaaaaaaaaa", Ivy is higher pitched while Salli is lower pitched.