Who, whom, which, that, what and whose are the relative pronouns. (more)
A relative pronoun is one of the two kinds of DMWs. When the DMW is a relative pronoun, it indicates that the dependent clause is most likely adjectival.
In Greek, the relative pronoun is declined for number, case, and gender. It is distinguished from the article by reason of the fact that it always has a rough breathing mark which the article usually does not. Study chapter 14 in BBG.