She is seen in the extended version of "iSaw Him First", after Sam lies to the school nurse, saying that her name was Rebecca Berkowitz. Rebecca was last mentioned in iSpeed Date, when a boy Carly wanted to ask out (Nate) said he was already going with Rebecca. Carly says Rebecca is "a lot of fun," and tells him to "enjoy her" in a weird tone.
It appears as though that Carly and Sam aren't very fond of her, for whenever they mention her, it comes as something negative.
She is also mentioned in a caption on the gallery of iSaved Your Life; the caption on a pic of Spencer in Sam's locker says "Spencer might have his head stuck in Sam's locker, but we think Rebecca Berkowitz has it worse. Her locker is on the bottom and Spencer has a very bad foot fungus."  She is portrayed by Katherine M. Ellis.
School Nurse: I'm going to report you to the principal for impersonating a sick person. What is your name?
Sam: Rebecca Berkowitz!
School Nurse: (shouting) You are in big trouble, Rebecca Berkowitz!
Rebecca: (scared) What did I do?