IDIOMS

straight man

(idiomatic) A member (of either gender) of a team of comic performers who plays a supporting role by helping to set up jokes and punch lines through engaging in preparatory dialog with the principal comedian; a foil who plays such a role in theatrical comedy.

Example: Used other than as an idiom: see straight,‎ man.
  the committee consisted of two lesbians and a straight man
  For many years Bud Abbott served brilliantly as Lou Costello's deadpan straight man.
2010, Caroline Graham, A Ghost in the Machine, Hachette UK (ISBN 9780755373222)
  Recently he had successfully concluded a case featuring a poet who wore only latex, lived on liquorice allsorts and worshipped a horse she believed to be the reincarnation of Radclyffe Hall. And she was the straight man.
1982, The Ladies' Home Journal
  She was the straight man, he was the comedian, and their audience was "standing room ...