In Friends, what is the name of Phoebe Buffay’s younger half-brother?

In Friends, Phoebe Buffay’s younger half-brother is Frank Buffay Jr.

Frank is introduced in the third season, when Phoebe discovers that Frank is her half-brother through their biological father, Frank Buffay Sr. The character is played by Giovanni Ribisi, who later became a recurring guest star on the series.

Frank is portrayed as highly intelligent but socially unusual. He studies science and eventually becomes involved with Alice Knight, his much older home-economics teacher. Their relationship is a major source of surprise for the other Friends characters, especially because Frank treats the age difference casually.

Frank Buffay Jr. is sometimes confused with Phoebe’s twin sister, Ursula, or with Phoebe’s romantic partners. Frank is her half-brother, while Ursula is her biological twin and Mike Hannigan is her eventual husband.