Jotaro Kujo (A Character Study)

Jotaro Kujo is a 17 year old, Japanese high school student, and is the main character of part 3. Even as teenager, Jotaro is established as being tall, attractive, and well built. He also bares resemblance to his great-grandfather, Jonathan Joestar, from his dark hair, strong jaw and light eyes. His hobbies consist of reading books about planes and ships, and his favorite movie Never Cry Wolf, could both play into his later occupation of a marine biologist.

When Jotaro is introduced he is initially portrayed as a blunt, foul-mouthed delinquent, who can be more than impudent towards anyone he finds annoying or simply dislikes, even his own mother. We are later shown that he has a gentle heart and is loyal to his friends and family.
Early examples of which are when he puts his life on the line to save Kakyoin, who had previously attacked him, and when he inquires on his mother's well-being.

Jotaro is a mostly quiet, cool headed character who shows little emotion, often expressing himself in short phrases and one-liners; which he often uses to ridicule his targets.
The best example being after pummeling Steely Dan, he tosses him a piece of paper listed with all the ways Dan had wronged him while saying, “And here… Is your receipt…”

There have been few times where Jotaro has lost his calm demeanor throughout the series.
During Jotaro's fight against Dio near the end of Stardust Crusaders he loses his cool after learning of his friend's death and seeing Dio drain Joseph's blood, ignoring his grandfather's warning not to get angry over his death.
Then again in his first encounter with Kira in part 4. After Koichi is almost killed by Kira, a heavily wounded Jotaro stands back up, being reminded of his fight with Dio as Kira talks down to him just as Dio had done previously.

For much of his youth, Jotaro was a normal kid, with the support of his mother while his father was constantly on tour due to his career as a musician. Although we aren't give an exact explanation of when and why, we can assume that his shift in personality is due in part to the absence of his father throughout most of his life.
Examples could be his trouble with expressing emotions other than annoyance and anger, the latter of which he expresses mostly through his stand. His smoking and drinking problems at age 17. And most notably in part 6 where he is divorced from his wife and is constantly away from home, not quite knowing what to do, as his own father was mostly absent and his grandfather not being the best example to follow. As a result he is rather detached from his daughter's life, only finally showing up when she is throw in prison for a crime she didn't commit.

Another reason for Jotaro's shift can be because of the appearance of his stand, Star Platinum.
Stands are explained as the physical manifestations of a person's fighting spirit and generally reflects the user in some way. This is no more true for Jotaro, as his stand, Star Platinum, is a reflection of his more violent side. Even Jotaro described it as being excessively violent. It's physical features are similar to Jotaro’s, if not more muscular, and is seen with a malicious grin when it first appears; later changing to a more stoic expression. Star Platinum’s design consists of mostly darker colors like purple and black. Later in part 4 it changes much of it's purple coloring to cyan and the black to white, symbolizing how Jotaro has become much less violent and more like a teacher figure to people Koichi and Josuke.

When faced with conflict Jotaro can be both fierce and gentle.
Examples can be found in pretty much every fight he takes part in throughout the series, as he usually opts for seriously injuring his opponents rather than killing them, even though he knows very well that they may come back to kill him.

Information sources: Jojo's Bizarre Adventure Part 3: Stardust Crusaders, part 4: Diamond is Unbreakable, part 6: Stone Ocean.