Second Banana is a term that has been around a really long time that meant "second billing" in an acting company. It's most often applied to comedians or comedies now; in such cases it's the supporting comedian, who may also be the StraightMan. The Second Banana may no longer actually get second billing.
In terms of characters, it is most often the sidekick, best friend, or co-worker of the protagonist.
* Luigi from the SuperMarioBrothers games. Webcomic/BrawlInTheFamily [[http://brawlinthefamily.keenspot.com/2010/05/27/248-bananas/ got a joke out it]].
* Art Carney was Second Banana to Jackie Gleason in TheHoneymooners.
* In TheAndyGriffithShow, and later in MayberryRFD, Don Knotts was the second banana.
* NeilPatrickHarris's Barney Stinson is the second banana on Series/HowIMetYourMother.
* Vivian Vance was usually Creator/LucilleBall's second banana in ILoveLucy and TheLucyDesiComedyHour.
* There was a character (a mouse) actually ''named'' Second Banana in some Warner Brothers cartoons, and in some Gold Key comics based on them. He was the sidekick of Merlin the Magic Mouse, a vaudeville-style stage magician.