The girl celebrated at a quinceañera is traditionally called the quinceañera.
The term comes from quince, meaning fifteen, and the feminine Spanish ending -añera in this cultural usage. In English, the word can describe either the birthday girl or the celebration, so context determines the meaning.
A quinceañera may be accompanied by a court, family members, sponsors, or friends. Other Spanish terms identify those participants: a quinceañero is traditionally a male celebrant, while a madrina is a female sponsor or ceremonial attendant in relevant contexts.
The word does not simply mean any teenage girl. It specifically connects the celebrant with the fifteenth-birthday tradition, whose details differ across countries and families.