Stephanie is 20 years old, and is a junior at Metrocity University in the early genetics degree (B.S.). She's reasonably focused on her studies, although becoming a super has her both wanting to continue her studies to be a role-model, but also to leave the college due to the problems with her telepathy when near groups of people.
Her powers include telepathy - she's primarily a powerful short-range receiver, as well as flight (not generated via mental control), and pheromones. The pheromones have both an emotional control component as well as directly allowing her to force people to do her will.
Stephanie strongly believes in being a hero and helping others, although her temper sometimes causes problems when things don't go well for her. She's a bit of a flirt, although is reluctant to let things go too far with anyone because she's not sure if they actually like her, or if her pheromones are causing them to do things they wouldn't normally do. (Although she loves the attention.) She worked out daily with Lance and Rael, and being in excellent shape made her somewhat of an exhibitionist even before the change. She is also trained in basic medical and martial arts skills, although she never took them as seriously as the boys did.
Stephanie's father is also a psionic super, although his exact power hasn't been revealed. He was instrumental in ensuring the kids were at the mall when the meteor shower exploded overhead, causing their change into supers. He was gone for most of her teen years, and left her in the care of Lance's mother while on his many trips.
I pronounce her name "sigh-star."
Psystar first appeared in H.E.R.O. - Metamorphosis.