@ScarecrowB The stars appear to be a sign of their power. Notice how all of the other girls have stars on their transformed formes (Leber's arm, Musica's neck, Gigant's eye, Enorme/Alice's forehead, Utena's cheekbones). Leo lacks any visible stars, so she was called a no-star.
@Pinka so the author is going the usual route where by defeating someone you gain their stars and become stronger? And since everyone has a different starting number is a way to determine their innate potential or something?