Jazz, being the older sister and feeling the most rational of her crazy family, takes it upon herself to be the "mature" figure, often criticizing her parents' obsession with ghosts and protecting her younger brother. While she may not know it, she often uses her intelligence as a means of superiority over 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. Simply put, she's more of an adult stuck in a teen's body. 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 an annoyance to others.

In The Fenton Menace, when Danny Fenton tears her favorite teddy bear apart, Jazz instantly starts to throw a fit over her stuffed animal, jumping up and down and crying.

Jazz eventually develops, starting off as a rather snobbish genius who refuses to be associated with her parents while maintaining a serious attitude, only for her to later discover how much fun ghost hunting is and embracing it, as well as toning down her serious nature and discovering her inner child while still being her supportive self. Her sudden increased interest in ghosts causes Jazz to imply or hint she is suffering from "ghost envy" when she states 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 attempts to support him.

She helps Danny by continuously covering his identity from suspecting eyes--particularly their parents. She is also terrible at giving ghosts names, and making puns.

She is apparently popular in her school and very social, and is as bad a singer as Tucker.