On "Friends," Ugly Naked Guy plays the cello.
Ugly Naked Guy is the unseen—or only partially seen—tenant whose apartment faces Monica's. The friends regularly watch him through the window, usually while he is naked and unaware that they are observing him. His cello playing is one of the recurring details that helps define the character beyond the joke in his nickname.
He is first mentioned early in the series, and the group later uses a giant poking device to check whether he is dead after seeing him lying motionless. Ross eventually tries to befriend him because the apartment becomes available, leading to one of the show's most awkward attempts at negotiation.
The character appears only briefly and is mostly discussed rather than shown. His real name is never important to the story, and for years viewers also wondered who played him. The role was eventually identified as actor Jon Haugen. The cello is easy to confuse with Phoebe's guitar, but Phoebe is the musician among the main friends; Ugly Naked Guy's instrument is specifically the cello.