"You are a person who is positive."
A few of us have answered wrong apparently as follows ... Tú eres una persona quien es postiva ...why is this wrong?
It doesn't even sound natural in English. I guess it could be literally translated to "Eres una persona que es positiva".
As a native speaker "Persona quien es /adjective/" just sounds wrong. It doesn't matter if it's masculine, feminine or plural, we always use "que". "La persona que tiene un gato", "El chico que tiene un gato" or "Los chicos que tienen gatos".
A proper use of "quien" would be "Quien tiene un gato, tiene un animal" (Whoever has a cat, has an animal). It is an indefinite expression. You are not naming any particular person, just everyone who has a cat.
"quien" is a interrogative pronoun, i.e. it is used in questions. In this context however, "who" is a relative pronoun and can be interchanged with "that", i.e. "You are a person that is positive." A relative pronoun is used to explain another part of the sentence, and thus you should not use "quien" but "que".
I agree, we use "que" to describe a thing or person already mentioned in the sentence. "un carro que es rápido" (a car THAT is fast), "una persona que es tímida" (a person THAT/WHO is shy).
We can use "quien" in sentences, when quien is the object of the verb, and we use it with "a".
Una persona A QUIEN [le] debo mi vida-- A person TO WHOM I owe my life.