This is the 8612th most frequent Chinese word.
who
The Chinese word for "who" (谁 shéi) is used to ask about a person or people.
In this sentence, '谁'(shéi) is used as an interrogative pronoun to ask about a person's identity.
谁是你的朋友？
Who is your friend?
'谁'(shéi) in this sentence functions as an indirect question word within a subordinate clause.
你知道是谁敲门吗？
Do you know who knocked on the door?
'谁'(shéi) is used as a relative pronoun to represent an indefinite person in this context.