second banana

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One, such as an assistant or deputy, who is subordinate to another.
• noun One who serves as the straight man opposite the leading comedian in a burlesque.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A comedian who plays a secondary or supporting role, especially as straight man and traditionally in vaudeville or burlesque theatre.
• noun idiomatic A person who serves in a supporting, secondary, or subsidiary capacity; an assistant.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who serves in a subordinate capacity or plays a secondary role
• noun a performer who acts as stooge to a comedian