Who’s your friend?

Who's your friend?

Who is this person accompanying you? A: "Mary, we're going to be having a meeting at 11 this morning. Who's your friend?" B: "Oh, this is my niece. She wanted to see where I work." A: "Hey, Sam. Who's your friend?" B: "How's it going, Mike? This is my pal Richard. He's visiting from out of town."
Farlex Dictionary of Idioms.

Who’s your friend?

interrog. Who is that following along behind you? JOHN: Hi, Tom. Who’s your friend? TOM: Oh, this is my little brother, Willie. JOHN: Hi, Willie.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • Who's your friend?
  • when he's, it's, etc. at home?
  • who's (someone) when (he's/she's/they're) at home
  • who's someone when they're at home
  • Who's on the phone?
  • big drink of water
  • studly
  • a tall drink of water
  • Look who's here!
  • Can I tell (one) who's calling?