Leslie is the middle child out of the triplets (her older brother, Frank Jr. Jr. and her younger sister, Chandler Buffay.) Her parents had trouble conceiving naturally (most likely due to Alice's age), so Frank asked his sister, Phoebe Buffay, as their wedding present, to be the surrogate of their child. Leslie was born, along with Frank Jr. Jr. and Chandler, in the episode "The One Hundredth".
They were seen again aged one in the sixth season, and aged five in the last season. Frank asks Phoebe to adopt one of them (similarly to "The One Hundredth", when Phoebe asks Rachel to ask Frank Jr. if she can keep one of the babies), but subsequently realizes he loves them too much to give one away.
Phoebe Abbott is not Leslie's grandmother because her father, Frank Jr, and Phoebe Buffay have different mothers (both adoptive and biological). Lily Buffay is not related to Frank Jr. by blood, and Phoebe Abbott is Phoebe and Ursula's biological mother, but not Frank Jr.'s. Frank Jr. is the child of Frank Buffay Sr and an unnamed woman.
- No one seemed to notice that Leslie is the name of Phoebe's ex-singing partner.
- In the episode "The One With Joey's Porsche" it becomes obvious that the triplets are portrayed by quadruplets, where only one is a girl (when Leslie is seen sitting in a green dress, it is clearly visible that the person is a boy, but when Phoebe picks her up, it's a girl).