Determine the class of the italicized pronoun.
marcia is the girl who lives upstairs.

In the sentence, assuming that the italicized pronoun is ‘who’ is a first person class.  

A pronoun is used to substitute a noun. In order for it to substitute, it must have a clear antecedent. Personal pronouns are used to substitute nouns with ownership. There are three persons point of view.1st person is when the subject is the one who is speaking (e.g. I, me, my, mine, we, us, our, ours). 2nd person is when the subject is the one being spoken to (you, your, yours). 3rd person is when the subject is the one spoken about (he, him, his, she, her, hers, it, its, they, their, theirs).