Danny may have the superpowers in his family, but Jazz has the brains and social skills. She starts off the show as the intelligent older sibling who believes her parents' "sick obsession" with ghosts is silly. She is often overprotective of her brother Danny to the point of annoyance and views herself as the most normal member of the family.
Jazz eventually accepts that ghosts are real, sentient beings when she discovers Danny's ghostly powers by seeing him transform behind the Nasty Burger. Danny, however, is unaware that she has witnessed him transform. At first, Jazz tries to question Sam and Tucker, Danny's best friends, but she quickly gives up once she realizes that they won't betray Danny's secret. When Danny rescues her the next day while in ghost form, Jazz pretends to be scared of him and runs away. Smiling, she tells herself that "he can tell me when he's ready." As a result, Danny has no clue that she now continuously covers for him when he's out capturing ghosts.
Jazz gradually changes in her views of ghost hunting when she is forced to accompany her father in barricading their own home, protecting it from a massive ghost invasion sent by Vlad Masters to murder Jack. In spite of her assurance to her dad that ghost hunting is nothing but a social experiment, she finds herself enjoying it, easily adapting to the Fenton equipment. Ever trying to be mature, Jazz eventually embraces her childhood, proving that she doesn't need to be the adult all the time and ultimately creating a balance between the two.
While Danny is reworking a desolate timeline so he won't become Dan Phantom, Jazz sends him a note ten years in the future to inform him about Vlad. Danny recognizes her handwriting and her trademark headband used to secure the note, informing him not just about Vlad, but also that she knows about his double life. After defeating Dan, Jazz confesses to Danny, telling him that she's known for a long time and was waiting for him to reveal his secret on his own terms. Danny and Jazz, in a touching moment, embrace, finally able to reconnect.
Jazz begins to try and take a more active role in Danny's ghost fighting, and although she means well, her inexperience results in more than a few embarrassing defeats for Danny, such as accidentally sucking him into the Fenton Thermos multiple times. Danny eventually loses his temper with her about her unnecessary help, hurting her feelings. To prove Danny wrong, Jazz ventures to the home of Vlad Masters in hopes of helping Danny out, but ends up forced into combat with Danny. After working together to defeat Plasmius, the two smooth out their conflict and Jazz reverts back to her old method of covering up for him and only helping when she's needed. In gratitude, Danny dubs her a member of Team Phantom, a role she plays more fully when she teams up with her mother, Maddie, and Sam to defeat Spectra, Ember, and Kitty when they overtake Amity Park.
With her new-found love of ghost hunting, she expresses it as part of her social life, even going so far as writing her college entrance thesis on "ghost envy". At the end of the series, Jazz reveals the truth about Danny's ghost powers to their parents when she thinks he's been killed in a crash. Later, when he turns up in one piece, she explains to Danny that she told his parents about his secret.
Jazz, being the oldest sibling and insisting that she is the most rational member of her family, takes it upon herself to be the "mature" adult, often criticizing her parents' obsession with ghosts and being overprotective of her little brother. She's an adult stuck in a teenage body and wants to be a good role model for her younger sibling, and is often concerned about his development and well-being, much to his annoyance.
While she may not consciously know it, she tends to think that her intelligence makes her superior to others, going completely bonkers if she's shown to be wrong and often using her older-sister status to boss Danny around, acting more like a mother than a sister. Ultimately, she means well, using her knowledge to point her parents in a more "normal" direction and keeping an overprotective eye on Danny, even if it sometimes is quite annoying to others.
Although she is an adult stuck in a teenage body, she does have one weakness: her favorite stuffed teddy bear. In The Fenton Menace, when Danny Fenton tears the little bear apart, Jazz throws a fit, jumping up and down and crying.
Jazz gradually develops as the show progresses, starting off as a rather snobbish genius who refuses to be associated with her parents, only to later embrace the family ghost-hunting trade. She also learns how to tone down her serious nature and discover her inner child. Her sudden increased interest in ghosts causes Jazz to imply that she is suffering from "ghost envy", stating she desires to be a ghost to get her parents attention. In addition, she develops a deep respect for her brother's selflessly heroic nature and supports him through thick and thin.
At school she is apparently popular and very social. She is one of Casper High's best students according to Mr. Lancer and gave a school spirit speech that was applauded and cheered on by everyone in attendance.