The show introduces Mouse as the best friend to the protagonist of the show, Carrie Bradshaw. Mouse is a loyal friend to Carrie, but also annoying, and when she sees Sebastian Kydd at the country club with Carrie's arch nemesis, Donna LaDonna, she quickly calls Carrie to let her know. She admits in the first episode to having sex with a "Princeton guy," Seth, during the summer at a camp. Her "boyfriend" eventually stops calling, and she admits to calling him too many times. He does call back, and she says she misses him. In "Read Before Use," she shares that her parents came to America so she could have a good education. She goes to a party at the Franklin Furnace with Seth and Carrie, and by the end of the date, Mouse and Seth are back together. When Carrie goes to a party for Halloween at Larissa's, Mouse, a loyal friend, goes to spy on Carrie's ex-boyfriend, Sebastian at his party. Her competitive side gets the best of her, and she ends up befriending Sebastian over the game of Pac Man. However, at Sebastian's party, Mouse has marijuana for the first time, and Sebastian has to take her over to Carrie's because she is so stoned.
In "Dangerous Territory," Mouse begins worrying about why Seth and her have not had sex since they got back together. She believes she is bad at sex due to Maggie's advice. She asks Walt for advice, and he comes over and watches a "how to" video on sex. Afterwards, Seth believes she has been with someone else because she is so great at sex. So, he admits that he's been with two other people while they were broken up, and Mouse pretends that she has also. Seth has a double standard and cannot believe that Mouse slept with someone. He demands a name, so Mouse lies and his name is Walt. Seth and Mouse run into Walt and Maggie at the diner. When Mouse introduces him to Walt, Seth tries to attack Walt, so Mouse tells the truth. In "Endgame," Carrie ruins her Thanksgiving dinner, so Mouse comes over to help and ends up doing most of the work. In "Caught," Mouse gets her first B+. She blames this on her relationship with Seth. She does extra credit, but finishes it too late. She breaks up with Seth because it is ruining her grades. In "Hush Hush," Mouse skips the junior-senior lock-in to go to Manhattan to help Sebastian find Carrie. In "The Great Unknown," she meets Thomas West, a smart, athletic African American with a single-parent mother. She discovers that due to her B+, she is no longer top of her class, and West has replaced her. In order to prove herself to West, she challenges him to and wins a Rubik's Cube off, but West does not care.
In "The Long and Winding Road," Mouse meets up with a Harvard graduate, Stacy Ling, to talk about college. On Mouse's tour of the school, Stacy meets West and is very charmed by him. West invites Stacy to the spirit night, and Mouse also decides to go to chat more with Stacy. Stacy admits that it will be hard for Mouse to get into Harvard because she has no "hook." When the football team is impressed by her organizational skills, she decides to become the basketball manager to find a "hook." She takes her new job very seriously. However, she realizes that a team needs to work with each other, not against each, and the rankings she showed them did the opposite. Learning from Donna, she purposely gets fired, so the team can have a common enemy. In "Identity Crisis," she begins to see West as a cute boy, not competition. After being fired, she becomes determined to have ten leadership roles in the school. Although everyone leaves her "USA For Africa" Club, West stays. Mouse denies her attraction to West, but she ends up sleeping with him. Mouse continues to deny this connection, but West proves that they should be together, so they date. Out of fear of her parents' disapproval and racism, she does not tell them about West. Her parents find her a suitable, Chinese date for the prom, an 8th grader who still has braces. Mouse's main reason for obeying her parents rules is because she believes she is Chinese royalty. However, she discovers from her prom date that she is from a line of peasants, not kings and queens. After this discovery, she tells her parents about West.
When Mouse learns how Maggie kissed Carrie's boyfriend, Sebastian, she refuses to speak to her. However, Maggie becomes her and West's waiter at the country club for the Fourth of July. Mouse is very rude to Maggie, so Maggie switches tables, and someone else becomes Mouse's waitress. Mouse, appalled that Maggie is not sucking up to her, confronts Maggie. Unfortunately as they argue, Mouse continues to back up almost into a cake with lit sparklers until Maggie warns her. She instead falls into the pool. Maggie and her talk in the locker room, and Maggie has gotten fired for arguing with a member. Mouse believes that Maggie got the job just to see hot guys, but learns that Maggie needs it for college. They become friends again.
In "Express Yourself," Mouse is at a camp when West calls her and tries to have "phone sex." Completely mortified, Mouse hangs up. When Mouse goes to Manhattan to stay with Carrie, she tries to learn from Carrie's roommate, Samantha, how to do it. She fails miserably. She bakes a quiche for Walt's birthday, but it turns out the eggs she used were expired, and she got everyone food poisoning.
The Mouse is an overachiever. She is very intelligent and strives hard to be number one. She has a hard time balancing her relationships and schoolwork, this leads to her breaking up with her first major boyfriend, Seth, in order to protect her school standing. She is very determined to get into Harvard, believing (in the beginning) that being accepted into an ivy league is the most important thing in her life. She also had the tendency to be shy around people who weren't in her close-knit group of friends at first. She later becomes more open and social when she starts her first major relationship and then discovers that, to get into Harvard, she must join more clubs. She has problems with her lack of popularity until, with help from her second major boyfriend, Thomas, she realizes that it is the quality of her friends that matters, not the quantity. She, like Carrie, is also big on trust and loyalty. This is shown when she stops talking to Maggie after Maggie betrays Carrie. She also is compassionate and later helps Maggie when she discovers that Maggie has been having a hard time and is thinking of not going to college. Mouse believes that with hard work and hope anyone can achieve their goals.
The Mouse has a black bob for the first season. Eventually her hair grows out to her shoulders. She is rather pretty and small, thus her nickname (Mouse). She always wears preppy sweaters and ties in school and doesn't dress down very often, she believes in maintaining a professional appearance. She is of Chinese descent.
- Seth is in either his first or second year of Princeton University. Mouse and Seth meet over the summer while she is at a camp at Princeton. In "Pilot," Mouse confesses that she had sex with him over the summer. However, she does not hear from him for a couple of weeks. She calls him "too many" times, and she does not hear from him. Finally, he calls back. She does not know if she misses him or not, but they meet up in Manhattan. Although she just wants to get ice cream with him, her friends convince her that she must go to a club or do something exciting. So, Mouse and Seth join Larissa and Carrie at a club. Seth feels slightly uncomfortable at this club because its focus is the power of a woman's vagina. They meet outside and recognize that they both do not want to be here. They continue to date, and Mouse realizes that they might have broken up in the first place because she is bad at sex. Walt and her watch a "how to" video, and she becomes very good at sex. Therefore, Seth is convinced she
- Mouse meets West in the hallway one day when she is talking about the difficulty of Rubik Cubes. He takes her cube and quickly completes it. She at first thinks it's just a fluke and that he can't really be that smart because he is a jock. She later discovers that West has replaced her as the top of the class. Mouse becomes worried about whether she will be admitted to Harvard because, as Thomas points out, Thomas is a genius African American three sport varsity champion from a single family whereas Mouse is a generic smart girl with nothing that makes her stand out. Because of this Mouse begins competing against him, all the while falling in love with him (which she refuses to recognize). Thomas, knowing and accepting his feelings for Mouse, revels in the attention and continues to bait her into situations where she must interact with him (leading her to question her feelings for him). At the "USA for Africa" meeting, she ends up sleeping with him although she still denies her attraction afterwards. Exasperated by Mouse's denial of her true feelings, he calls a truce and then convinces her that he can help her do better in school, and even gets her a good recommendation letter from a teacher. Because of this they begin dating officially, and Mouse even tells her parents about him despite the fact that he is not Chinese, showing that she really cares for him. In "the second time around" mouse and her boyfriend BREAKUP due to the fact that he was sick of the constant competition between them.
- In the book, her name is Roberta Castells. In the show, her name is Jill Chen.
- In the book, she loses her virginity to a "Danny." In the show, she admits to losing her virginity to Seth, a guy at Princeton.
- In the show, she dates Thomas West, who is a smart and athletic guy in her class. In the novel, however, there is no West.
- In the show, she must go to the prom with an 8th grade boy to honor her family's Chinese traditions. This does not happen in the book.