Party-Girl Sister is one of the main characters in "Rules of Engagement" series. Her suggested name is Jess but the player can choose a name for her. She is first seen in Book 1, Chapter 1. Her task is to keep one job for the entire summer.
Party-Girl Sister has long, brown hair, brown eyes and tan skin. Usually, she wears a white crop top with a pink flowery shrug and a chain with stars and a lot of make up.
Party-Girl Sister is a fun loving girl who likes to party and live to the fullest. She suffers because people think that all she can do is partying and can't take anything seriously. However, she shows that she can do anything she sets her mind to, as she can earn Blake's trust and respect. She will do anything for Bookish Sister, her twin, as shown when she pretends to be her for an interview.
Cousin has always been jealous of Party-Girl Sister and her siblings because she thinks that Nana loved them more than her. Now, after Nana's death, she tries to get the whole inheritance for herself by sabotaging her cousins' tasks.
Party-Girl Sister and Carter are co-workers. They even hook up together. In Book 2, it's shown that Carter is starting to fall for Party-girl Sister and is very disappointed and hurt as he finds out that she and Blake slept together.
Party-Girl Sister takes an instant liking to Blake at the Wine and Mixers when she sees him drinking her favorite whiskey. Later, she finds out that he's her new boss and she tries hard to impress him. Even though he doesn't know her very well, the manager sees more in Party-Girl Sister than other people do. He thinks she's smart and able to do more in her life than just partying and doesn't want her to give up on her job.
- Throughout several episodes, she has been craving olives, orange slices, pickles, and chocolate. It is seeing the olives and orange slices that makes her brother ask her if she's pregnant. It's the same thing their mother craved when she was expecting Party-Girl Sister and Bookish Sister.
- As revealed in Book 3, Chapter 7, Bookish Sister is the second youngest granddaughter of Nana, making Party-Girl Sister the youngest granddaughter, as well as Bookish Sister's younger twin.