Frank Buffay Jr.

Frank is Phoebe's half brother by their father. He first appears in "The One with the Baby on the Bus" as a stranger who leaves a condom rather than money in Phoebe's guitar case when she is singing in the street, and then comes back to retrieve it.
In "The One with the Bullies" Phoebe meets him after finding the courage to knock on her father's suburban door, but discovers he walked out several years ago. Frank visits the city in "The One with Frank Jr" where he thinks Phoebe works as a whore. In this episode we can see that Frank Jr. is a little... weird... the conversation between Phoebe and Frank is soooo interesting... or, at least, funny.

PHOEBE: So, um, what kinda things do you like to do at home?
FRANK: Melt stuff.
PHOEBE: Okay so, by melting, you meant... melting.
FRANK: Yeah.
PHOEBE: So is it like art?
FRANK: Yeah, you can melt art... Hey, can I use your phone?
PHOEBE: Um, yeah sure. Why you wanna call your Mom?
FRANK: No, I wanna melt it.

In "The One with the Hypnosis Tape", he announces his engagement to Alice Knight, his old home economics teacher and 26 years his senior.

Talking about Alice:
Joey: You're 18, she's 44. When you're 36, she's going to be 88!
Frank Jr.: You don't think I know that?

In "The One with Phoebe's Uterus", they ask Phoebe to be a surrogate mother to their child, which turns out to be triplets in "The One Hundredth."
Frank makes a final appearance in "The One Where Ross is Fine", when he and the triplets meet Phoebe at Central Perk. Here's the video with the las appearance of this surreal character: