Jolyne Kujo is the protagonist of part 6 of JoJo's Bizarre Adventure, the manga created by Hirohiko Araki that follows the story of the Joestar family throughout several generations - and 34 years of publishing. JoJo's Bizarre Adventure is divided into nine story arcs (at the moment), each with a new protagonist bearing the "JoJo" nickname, but Jolyne is fundamentally different from her predecessors and successors, and better for it.
Part 6 of JoJo's Bizarre Adventure, titled "Stone Ocean," is almost entirely set inside the fictional Green Dolphin Street Prison, in Florida. Jolyne is the estranged daughter of the protagonist of part 3, Jotaro Kujo, and is sent to prison after being framed for murder. It is later revealed that this was part of a scheme to lure her father and steal his memories, hatched by prison priest Father Pucci, who is a follower of Jotaro's nemesis from part 3, DIO. Jolyne then has to use her newly discovered "Stand" abilities (the physical manifestation of a person's life energy) to recover her father's memories and foil Pucci's plan to "send the world to Heaven."
Besides being the first, and to date the only, female protagonist of JoJo, Jolyne stands out for a series of other reasons. Previous (and following) protagonists fit firmly into the archetype of the traditional hero, defined by their purity of heart, the strength of their beliefs, and their inability to fail. The legacy and the true power of the Joestar family, in fact, consists of the values that run in their blood. Their sense of justice is the reason they are always able to step in front of evil and fight it, even at the cost of great sacrifice. Jolyne, on the other hand, is not exactly "pure of heart." Stone Ocean begins with her committing a heinous act: while in a car with her boyfriend Romeo, he runs over a pedestrian and asks Jolyne to help him get rid of the body.
It turns out that the man wasn't dead, so Jolyne is accused of murder. Even if it is later revealed that the man they ran over was also part of Pucci's plot, and even if her actions were motivated by love and the desire to protect her boyfriend (who still ends up betraying and framing her), it doesn't change the fact that Jolyne is guilty of a hit-and-run. A true Joestar would have turned Romeo in to the forces of justice, especially after his plea to hide his crime. As a consequence, Stone Ocean is actually a journey of redemption for Jolyne.
This theme is further consolidated by the fact that, contrary to other JoJo's, Jolyne is very fallible. She makes mistakes: it is her fault Jotaro's memories are stolen in the early chapters of the manga, putting her father into a coma and giving Pucci the details he needed to fulfill his plan. These mistakes are what motivate Jolyne into growing from the spoiled, shallow girl she is in the beginning to a person worthy of the Joestar name. This sets up an interesting dynamic that also plays into the themes of growth and self-development that are central to part 6. The final and most important difference is that, in the end, Jolyne fails. Even if Pucci is ultimately defeated and killed, he still enacts his plan to accelerate time on Earth to the point of singularity, killing Jolyne and her friends in the process, even if their counterparts are later reborn in a new universe as alternate versions of themselves (it is called a "bizarre" adventure for a reason.)
In the end, Jolyne's failures and shortcomings are what make her more "human" compared to the other protagonists of JoJo's Bizarre Adventure - while Jolyne shows the spirit of the Joestar family time and time again during "Stone Ocean," she also possesses different traits that make her less of a paragon and, in the end, more relatable.