Phoebe discovers Frank on one of her attempts to find her father Frank Buffay Sr., who walked out on her and her twin sister when they were children, and later also walked out on Frank Jr.'s family.
Frank Jr. has a punctured lung, was arrested for stealing birds' eggs, likes to melt things, and used to own a dog named "Tumor". He also believed that 'massage parlor' is a euphemism for brothel, wrongly presuming that his sister Phoebe was a prostitute.
Meeting his sister
Frank Buffay Jr. was the result of his father Frank Buffay Sr.'s marriage to his second wife, whom he also abandoned. Frank didn't realize that he had two older half-sisters, Ursula and Phoebe Buffay.
In "The One With The Bullies", they meet when Phoebe returns his dog after she hit it and reveals to be looking for Frank Sr. but is told he ditched them. Frank Jr. talks with Phoebe and learns they are half-siblings through the same father. While Phoebe did not find her father, she found a brother and they bonded because of their similarities.
During "The One With Frank Jr.", he visits his sister in New York, however they seemed to have trouble connecting as they have little in common, and ends with him groping one of her co-workers at her massage parlor, mistaking 'masseuse' to mean 'hooker'. However, this is cleared up and they reconcile after.
Frank starts dating his home-economics teacher, Alice (Debra Jo Rupp), in the third season. The relationship would have been a perfectly normal one were it not for the fact that she is 26 years his senior (she's 44, Frank Jr. is 18) and he often refers to her as Mrs. Knight instead of using her first name. Despite the age difference, the two are definitely attracted to each other.
He introduces Alice to Phoebe as his fiancée (in "The One With The Hypnosis Tape"), and reveals to his half-sister that he and his girlfriend are planning to marry and have kids right away. Finding the relationship disturbing, Phoebe does all she can to break them up, bemusing both of them with her daft comments throughout. But they're too much in love to listen to her, and Frank Jr. has difficulty listening to Joey and Ross when they try to talk him out of the relationship. During Ross and Joey's conversation with him, he inadvertently convinces them to support his decision when he counters Joey's argument about the limited amount of women he's slept with by asking if sex isn't better when it's with someone you really care about, prompting Ross to remember his past relationship with Rachel and Joey to express envy that he hasn't experienced a relationship like that, culminating in Joey agreeing to be Frank's best man and Ross to be the ring bearer (Although neither were needed in the end).
Marriage and kids
They get married in "The One With Phoebe's Uterus"; while giving Phoebe the good news, they ask her a favor: they wish her to carry their child. Phoebe discusses this with her birth mother and decides to be Frank and Alice's surrogate. One episode later, Phoebe has the embryos implanted in her womb and she gets pregnant. The pleasant surprise gets even better for Frank and Alice when they learn that Phoebe's pregnant with triplets.
The pregnancy marks Phoebe's life in several ways. She becomes dismayed when she, a vegetarian, begins craving meat, warring between her principles and her cravings, moving Joey to give up meat because then the same amount of animals must die. Phoebe accepts this and eats Joey's share of meat. ("The One With The Fake Party") She unleashes her hormonal anger on Monica and Rachel ("The One With The Worst Best Man Ever") and she misses Ross' wedding in London due to not being able to travel by airplane in her third trimester.
In "The One Hundredth", Phoebe reveals that she wants to keep one of the kids for herself, and asks Rachel to ask Frank Jr. for the favor. Rachel is reluctant to do so and tells Phoebe that Frank and Alice want all of their children. Phoebe eventually gives birth to the triplets: Frank Jr. Jr. (named by Frank Jr.); Leslie (named by Alice); and Chandler (named by Phoebe), who turns out to be a girl, but Frank Jr. and Alice name her Chandler anyway.
Frank shows up in the tenth season in "The One Where Ross Is Fine". He admits to Phoebe that he and Alice are finding it hard to raise their three children (he says he hasn't "slept in four years"), and asks Phoebe if she wants one of his kids. However, it's impossible for him to get rid of any of his kids. Phoebe understands the position he and Alice are in and explains to Frank that she'll always be available for baby-sitting should the need arise.
- Giovanni Ribisi has also appeared in "The One With The Baby On The Bus". However, he is a stranger on the street who had dropped a condom in Phoebe's guitar case when she was playing "Smelly Cat" on the street. He comes back for the condom because he needs it "for an emergency". "Later" in the same season, Giovanni appears as Frank Jr. - although it is not confirmed whether the two characters are one and the same -- and Frank Jr. claims that it is the first time he came to visit New York city.
- It's not clear if Phoebe calls Frank after finding their father in "The One With Joey's Bag" because she is seen calling Ursula.
- Pheebo was originally Frank Jr's name.
- It is unknown if he is aware of Ursula.
- He has a tendency to add "you know" to many of his sentences, much like Phoebe.
- "The One With The Baby On The Bus" (First appearance)
- "The One With The Bullies"
- "The One With Frank Jr."
- "The One With The Hypnosis Tape"
- "The One With Phoebe's Uterus"
- "The One With The Embryos"
- "The One With The Free Porn"
- "The One Hundredth"
- "The One With Joey's Porsche" (Mentioned only)
- "The One Where Ross Is Fine"