To respond, you need more context. What is the situation? Here are some ways you could respond, depending on the context:
If you know the girl:
* "Esa es [nombre de la muchacha]." (That's [girl's name].)
* "Ella es mi amiga." (She's my friend.)
* "No la conozco." (I don't know her.)
If you don't know the girl:
* "No lo sé." (I don't know.)
* "No estoy seguro." (I'm not sure.)
* "No la he visto antes." (I haven't seen her before.)
If you want to be more specific:
* "Es la hija de [nombre]." (She's the daughter of [name].)
* "Ella es la nueva estudiante." (She's the new student.)
* "Ella es la que trabaja en la tienda." (She's the one who works at the store.)
Remember to choose the response that best fits the situation and your knowledge of the girl.