Sergei is a diplomat who Phoebe dates.
He does not speak a word of English and has to have his translator, Mischa coming around with them which irritates Phoebe.
Phoebe invites Monica to date Mischa and she accepts.
At a restaurant during their dates, Monica and Mischa are doing quite well together, but then a fight between Mischa and Sergei ensues and Mischa resigns his post and leaves with Monica who accompanies him to the Rainbow Room. After they leave, Sergei picks up a plate and says "Plate." which makes Phoebe pleased but then he picks up a cup and says "Plate?" and Phoebe fakely answers "Yeah."
During the credits, Sergei is at Central Perk with Phoebe and he tries to teach her how to sing American Pie.
He keeps going "Amer-ican" then shouts to everyone in the coffee house "Everybody!" in his language and continues playing.