Abby is Jess' older sister who, according to Jess, has never really found footing in life and is always drifting. Abby considers her life an endless party and adventure, and in many ways Jess envies Abby for her spontaneous nature - although Abby does feel like she's adrift.
Abby gets arrested in a hotel in San Diego, and Jess rescues her. Jess then goes on a mission to keep Nick from meeting Abby. Jess is not too thrilled by her sister. She hasn't said it, but Jess subtly mentios that she's embarrassed by Abby. However, she understands Abby is in need of assistance, therefore she allows her to stay in her room until she "gets on her feet". When Schmidt meets Abby, they begin messing around. At the end of "Sister III", Abby leaves the loft and begins living with her mother back in Oregon.