second banana

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

Also, top dog. The principal person in a group, organization, or undertaking, as in His plan was to be top banana within ten years, or Now that she's top dog you can't get hold of her at all. The first term comes from show business, where from the early 1900s it has signified the leading comedian (possibly the original allusion was to Frank Lebowitz, a burlesque comedian who used bananas in his act). It also gave rise to second banana, for a supporting actor, usually a straight man. Both were transferred to more general use in the second half of the 1900s, as in executive Peter Barton's statement, "There is a certain pain to being a second banana, but you have to have an ability to sublimate your ego," quoted in The New York Times, May 15, 1996. The variant, top dog, originated in sports in the late 1800s and signified the odds-on favorite or winner in a contest; it alludes to the dog who wins (comes out on top) in a dogfight.