Jennifer Tallulah Humphrey is one of the characters in both the "Gossip Girl" and "The It Girl" series of novels by Cecily von Ziegesar. She is portrayed by Taylor Momsen in the "Gossip Girl" television adaptation on The CW.
Novel series.
"Gossip Girl".
Jennifer Humphrey is the daughter of Rufus Humphrey, an editor of Beat poets who has never been published himself, and Jeanette Humphrey, who ran off with a European aristocrat. She has an older brother, Dan Humphrey, an aspiring writer. Jenny is a student at the Constance Billard School for Girls, a small, elite, all-girls school on the Upper East Side that Serena van der Woodsen and Blair Waldorf also attend. She is also very interested in art, which is shown when she briefly dates Nate Archibald and in turn paints several portraits of his face. After being expelled from Constance, Jenny is able to convince her father, Rufus, to let her attend Waverly Academy, a boarding school located in upstate New York. 
In the book series, Jenny is described as being very short (around 5'0"), having curly brunette hair, and a herbal-enhanced Double-Ds, but the television show depicts her as a being tall and boney, with long and straight blonde hair. In the "Gossip Girl" prequel "It Had to Be You", Jenny is noted for having a rather flat chest until she begins taking breast enlargement supplements, which are the cause of her DD-cup size.
"The It Girl".
Jenny is the protagonist of the book series "The It Girl", which shows her new life after she gets expelled from Constance Billard and starts at Waverly Academy in upstate New York. In "The It Girl", Jenny tries to reinvent herself as a "popular and sophisticated city girl" and leave behind the person she was at Constance. When Jenny arrives at Waverly, she finds the students even more attractive, athletic, and intimidating than she had imagined. Jenny then meets her two new exceptionally beautiful and popular roommates: lithe and blonde Southern girl Callie Vernon, and red-headed, green-eyed, sophisticated New Jerseyan Brett Messerschmidt. She quickly learns that she has replaced their old best friend, the extremely attractive and notorious Tinsley Carmichael. Jenny can't help but feel that sleeping in such a lucky bed must rub off.
TV series.
Season 1.
Jenny Humphrey is introduced in Season 1. A pretty, blonde freshman at Constance Billard, she tries desperately to fit in with Blair's clique, but Blair makes it difficult for her, subjecting Jenny to the rules of high school hierarchy. A possible friendship ends when Blair discovers Nate's lingering feelings for Serena and Jenny's interest in Nate, entailing further cruelty to Jenny. When Blair's affair with Chuck Bass is exposed, Nate approaches Jenny and Blair's affair is made public. Blair's friends shun her for her hypocrisy and establish Jenny as the new Queen Bee of Constance Billard. Despite being one of the school It Girls, she is driven to prove herself. Lacking self-confidence because she is not as rich as the other girls, she sells her sewing machine and barters an expensive dress she stole from one of her friends. When Blair throws her a surprise birthday party, the other girls discover that Jenny stole the dress, bringing Blair's scheme to fruition. Jenny retaliates by bringing Nate with her to Blair's victory party and winning the girls to her side. Jenny and Blair then struggle for the position of Queen Bee. Jenny thinks she's found true love with Asher Hornsby, but their romance is short-lived when she discovers that he is gay. He convinces her to say that she lost her virginity to him in order to dispel the rumors that he is gay, promising that as long as she pretends it's true, he'll give her privileges that the Upper East Side can offer. But Blair and Eric out Asher at his own party, and Jenny confesses that she lied about having sex with him. Blair's clique abandons her once again, and she later finds Blair to tell her that she's won as it's "not worth it". In the season finale, Jenny receives an internship at Blair's mother Eleanor's company.
Season 2.
At the beginning of season 2, Jenny spends her summer working hard as a new intern for Eleanor Waldorf Designs. She apologizes to Eric for her behavior the previous season. He forgives her and invites her to the White Party in the Hamptons, where he introduces her to Tinsley Mortimer and she proves her worth to one of Eleanor's supervisors.
Eleanor fires Jenny after a critical remark about one of her dresses, but during a citywide blackout, Eleanor realizes her talent and hires Jenny. After constant harassment by Blair's minions, Jenny decides to skip school to pursue a future in fashion.
Blair and Serena have another fight about Serena's growing popularity, and Jenny is dragged into it during Eleanor's fashion show. When she changes the seating arrangement Blair had planned, Blair retaliates by telling Rufus that Jenny has been skipping school, and he confronts her. Blair attempts to sabotage Jenny's work in the show but Jenny improvises, proposing that Serena and the socialites walk the runway. In an effort to ruin Serena, Blair switches the final dress with one that Jenny made, drawing Eleanor's ire. The dress is a huge hit with the audience and the fashion show is a big success. Jenny realizes Blair's frustrations with her friendship with Serena, commenting that they have worked hard for what they’ve wanted and Serena just glides through. Eleanor later praises Jenny for her work.
Rufus eventually agrees to Jenny being home-schooled after seeing how committed and good at her job she is. At work, Jenny befriends Agnes Andrews (Willa Holland), a model who convinces her to start her own fashion line. Realizing that working for Eleanor won't help her develop as a designer and that Eleanor has begun to take advantage of her talents, Jenny leaves. Jenny also begins a short relationship with Nate when they share a passionate kiss after he rescues her from being taken advantage of by an older photographer. Jenny and Rufus argue over her quitting Eleanor's and Jenny moves out of the Humphreys' apartment and moves in with Agnes, who suggests that they plan a guerrilla fashion show at the charity gala honoring Lily and Bart. The show is a big success but Vanessa witnesses her kissing Nate, thereby straining their friendship. Rufus tries to get her arrested but is stopped by Lily. Agnes's fiery temper and their growing disagreements over the clothing line make it hard for them to close a business deal. Jenny steals Agnes's contact list, attempting to make a deal by herself. Upon learning of Jenny's betrayal, Agnes burns all her dresses and kicks her out of her apartment, leaving her with nothing.
Upon learning that she is too young to start her own business, she attempts to file for emancipation, but Rufus refuses. On Thanksgiving, Lily finds the papers after discovering that Jenny has been hiding in their apartment with Eric, and warns Rufus. Rufus and Jenny reconcile after a little persuasion from Eleanor and she returns home. During the Snowflake Ball, Jenny learns that Vanessa and Nate are back together even though Vanessa was aware of Nate's feelings for her. Nate breaks up with her after she helps to humiliate Vanessa at the ball. Jenny returns to Constance and is forced to contend with Blair's minions.
During the season finale and Serena's hunt for Gossip Girl, Jenny decides to compete to be the next Queen Bee after Blair leaves. She almost drops out, but Blair gives her a touching speech about keeping her eye on the prize. In the end Blair chooses Jenny and crowns her queen of Constance.
Season 3.
After Jenny fails to bring about egalitarianism at Constance, she resolves to rule as Queen Bee with her own group of minions. With her new position and wealth, Jenny's social-climbing persona resurfaces and she begins to erase her former Brooklyn self, throwing away her homemade clothes and sewing machine. Worried that Jenny is turning into a typical Queen Bee, Eric and Jonathan attempt an intervention, but are humiliated by Jenny and her clique. Eric teams up with Blair to take down Jenny at the Cotillion Ball by sabotaging her escort. The plan backfires, and Jenny instead secures Nate to escort her, winning her the respect of the Queens at every prep school on the Upper East Side. As a consequence of Eric's scheming, Jonathan breaks up with him and a rift emerges in Jenny and Eric's relationship.
Chuck introduces her to Damien Dalgaard, son of the Belgian ambassador, who deals drugs. Liking his wild lifestyle, Jenny strikes up a relationship with him and helps him supply drugs to his customers while sharing in some of the profit, even stealing some of Lily's “headache pills”. Lily and Rufus find out about the drugs, which Damien covers up with a story about his father. Against Rufus's will, Jenny continues her relationship with Damien, who seems interested in taking it further. Jenny, who is a virgin, backs out at the last minute, causing Damien to break up with her, leaving her to face her family.
At Rufus's suggestion, Eleanor reemploys Jenny to help out with her upcoming fashion show. Jenny is happy to be back, but is shocked to learn that one of the models she’ll be working with is Agnes, who, after the show finishes, takes revenge on Jenny by drugging her and leaving her in a bar to be taken advantage of. Nate rescues Jenny before anything happens to her. Because of this, Jenny's feelings for Nate reawaken and she aspires to win him over. She tries to kiss him but he resists her advances because he sees her only as a friend and because he is dating Serena. Jenny attempts to sabotage Serena and Nate's relationship by lying to Serena and implying to Nate that she is having an affair with Carter Baizen. Jenny's attempts fail, and her relationship with both is left tenuous after Nate kicks her out of his apartment.
At a gala event where Serena's recently returned father William Van Der Woodsen is making a speech, a former client of Damien's approaches Jenny, angrily telling her the drugs she sold her (Lily's cancer medication) gave her a yeast infection. Rufus overhears this and grounds Jenny to the Humphrey apartment in Brooklyn. During her punishment, Jenny researches Lily's medicine and tells Chuck there is something very wrong with the medication Serena's father is prescribing her. It is discovered that William has lied to Lily about her condition and been giving her drugs that make her sick in a plot to win her back from Rufus. Seeing an opportunity to break up Rufus and Lily and return her family to the way it was before, Jenny attempts to sabotage Blair, Chuck, and Nate's plan to unveil William's deception. When William is about to be held accountable, she quickly runs to William and warns him, telling him the police are coming to get him, and urges him to escape. Back at the van der Woodsen penthouse she reveals that she warned William and says she wishes her family could go back to normal: "When I lived in Brooklyn, I may have had to ride the subway to school, and make my own clothes, but at least our family was happy". Rufus says the only thing that needs to return to normal is Jenny. Eric is hurt by Jenny's attempt to leave the family, and says that if she wants out, no one is forcing her to stay.
Jenny hits rock bottom after sending Gossip Girl a picture of Dan and Serena in bed together, jeopardizing Dan and Nate's friendship, Dan and Vanessa's relationship, and breaking up Nate and Serena's relationship. Blair confronts Jenny, telling her, “Nate loves Serena, Dan loves Vanessa — God knows why — and Chuck loves me. But you, Jenny? No one loves you except your daddy. And after what you pulled yesterday, who knows if that's even true anymore?” Emotionally distressed, Jenny goes to Nate's apartment but instead finds Chuck devastated over his break-up with Blair and in her state of vulnerability, Jenny loses her virginity to Chuck. Afterward, Blair arrives willing to take Chuck back and Jenny sneaks out. She breaks down in tears to Eric, telling him that everyone hates her