Yo la conozco; es una amiga. I know her (I recognize her); she's a friend
That would translate over to: "She is a friend"
Conozco a una persona
Ella es una maestra
Well, first of all, it should be "ella es una doctora" and it means "she is a doctor."
Ella es una chica mala
It depends on what kind of vet you mean. If you mean she a veteran, it's, "Ella es una veterana." If you mean she's a veterinarian, it's "Ella es una veterinaria." In Spanish, there is no foreshortening of those words; they say the entire word, all the time.
It means she is stupid but I love her/ care for her
Una amiga
When do you go to your friend's house?
You could say "Eres una bendiga como un amigo."
¿Es él una ella?
Ella es una mujer.