Jill Green is Rachel's youngest sister. After their father threatens to cut Jill off after she buys a boat for a friend, Jill comes to the apartment looking for Rachel after Leonard referred to her as the only daughter he was proud of. She attempts to start a new life, as Rachel did, but Jill ends up breaking the rules (shopping to excess thanks to her having memorized her father's credit card numbers) and dates Ross to spite Rachel.
However, Ross ended the relationship before it could really start after realizing that, if something happened between him and Jill, nothing could ever happen between him and Rachel; although he didn't know whether or not he and Rachel would ever get back together, he didn't want to know that they never could. This caused Jill to leave, as she didn't want to stay around with someone who sabotaged her every move, bitingly asking Ross if he was gay (based on his rejection of her even after she threw herself at him) before departing.
Despite conflicts, Rachel appears to get along with Jill far better than she does with Amy. According to Amy, since her appearance in the show Jill has gained weight around her "butt and face", but this has not been confirmed.
- In The One Where Rachel's Sister Baby-Sits, Amy tells Rachel that Jill has got fat, despite her being lactose intolerant.
- According to production reports, the original plans for Jill would have featured a six-episode story arc, but Jill left after only two episodes. Rumors that this was caused by on-set tensions between Reese Witherspoon and Jennifer Aniston were started by the National Enquirer around the time of the episodes' production, but have been consistently denied by the two actresses. (Witherspoon's departure may also account for the unconventional nature of the subsequent two-parter "The One That Could Have Been").
- In The One With Rachel's Other Sister, Rachel stated that Jill is her favorite sister.
- Before meeting her, Phoebe called her the spoiled sister, thinking of the other sister as "the one that bit [Rachel]".
- The One With The Two Parties (First mentioned)
- The One Without The Ski Trip (Possible mention)
- The One With Rachel's Sister (First Appearance)
- The One Where Chandler Can't Cry
- The One With The Baby Shower (Mentioned only)
- The One With Rachel's Other Sister (Mentioned only)
- The One Where Rachel's Sister Babysits (Mentioned only)