Donny Kerabastos

He's playing bowling with his friends

Theodore Donald Kerabatsos aka Donny (played by Steve Buscemi) is the tertiary protagonist a friend of The Dude and Walter. He's an avid, incorrupt bowler and frustrates Walter who often says "Shut the fuck up Donny!"
Donny appears several times at the bowling alley wearing different bowling shirts that may be 2nd hand.
At one point in the movie the Dude mentions Vladimir Lenin, which Donny mistook him for mentioning John Lennon, and starts quoting the lyrics "I am a Walrus," from The Beatles' I Am The Walrus, several times. Walter quickly becomes enraged by Donny's mistake and tells him to "shut the fuck up, Donny," something he famously says throughout the film, and corrects Donny on his mistake stating that the Dude was mentioning "V.I. Lenin, Vladimir Ilyich Ulyanov" and not John Lennon.