This is the 61st most frequent Portuguese word.
quem
"Quem" means "who."
Used as an interrogative pronoun asking about an identity.
Quem está na porta?
Who is at the door?
Used as a relative pronoun introducing a subordinate clause.
Não sei quem fez isso.
I don't know who did this.
Used as an indefinite pronoun referring to any individual meeting a condition.