Stargirl's old name is Susan Caraway.
Although "Susan" is the name her parents gave her, Stargirl has changed what she is called many times. She first renamed herself "Pocket Mouse", then "Mudpie", "Hullygully", then finally, "Stargirl". Stargirl says that she changes her name when the old one "doesn't fit anymore". She explains,
"I'm not my name. My name is something I wear, like a shirt. It gets worn, I outgrow it, I change it".
Stargirl reveals her original name while she is being interviewed by a panel of her peers on the Hot Seat, a student-run video show. Although in typical fashion, she does not seem to notice, the questioning from the panel has a hostile tone. One of the students asks accusingly, "What was wrong with the name your parents gave you?", and another demands, "So what do your parents think? Are they sad you didn't keep Susan?" Stargirl's parents, however, are apparently comfortable with her attempts to express her individuality in this respect, and she replies,
"No. It was almost their idea. When I started calling myself Pocket Mouse when I was little, they called me that, too. And we just never went back" (Chapter 13).