Ginny (no last name is ever given) is the old high school girlfriend of Jack Koster. She works as a waitress, though Theresa (for some reason) assumes she could be a cashier in Jack's father's store.
Ginny shows up one Friday night when Jack and some of his friends are having a party. Ginny is portrayed as not being aware of Jack's relationship with Theresa, and gets uncomfortable when the latter walks in on them together. Theresa goes up to her room to change into a sexy outfit to make Jack jealous, but meets Steve instead. Steve and Theresa head downstairs, where she assures Ginny that their relationship meant nothing.
Even though Theresa says she "felt sorry for Ginny" regarding the confrontation between her and Jack, Theresa also often looks down on Ginny's social class. She describes Ginny as "the poor waitress...waiting on tables all [her] life," sitting at home "hoping [she] could keep a college educated man." Later on, when Theresa becomes rich and famous, she thinks to herself, "Theresa calling Jack. How are you doing with Ginny?" Theresa's treatment of Ginny, in fact, is one of the reasons readers opine Theresa is an unlikable character.