straight man

Short definition from Wiktionary

Noun

1. 1
   A member (of any gender) of a team of comic performers who plays a supporting role by helping to set up jokes and punch lines through engaging in dialog with another comedian playing a more eccentric or otherwise more interesting character; a foil who plays such a role in theatrical comedy.
2. 2
   Used other than figuratively or idiomatically: see straight, man; for example, a heterosexual man.