Jonathan Joestar.
Nicknamed JoJo. Just a 12 year old boy minding his own business in 1880's Victorian England. One day, his father announces that Dio Brando is moving in with them, and Dio proceeds to push him out of his father's favor and ruin his life in every way possible. Jonathan, however, puts up with Dio, managing to get beyond it all and become an archaeologist. He researches a Mesoamerican stone mask that was in his father's collection, that, at first, appears to be a trap that kills the wearer. JoJo then finds out that Dio plans to kill his father and races to get an antidote from the evil Wang Chan with his newfound friend Robert E. O. Speedwagon. After getting the cure, the two men save JoJo's father and fight Dio, who, during the fight, shows that he has used the mask on himself and not died but become a vampire, and burns down Jonathan's house and escapes. JoJo and Speedwagon meet the Italian William Antonio Zeppeli, who teaches JoJo the vampire-killing fighting style Hamon (Ripple/Sendo) to beat Dio. JoJo, Speedwagon, and Zeppeli go to a European village and face off with Dio.
Joseph Joestar.
Grandson of the original JoJo. An unscrupulous fighter who uses any means necessary to defeat his opponent. During the course of Battle Tendency, he travels the world to keep the vicious Pillar Men from obtaining the Red Stone of Aja and attaining a level of immortality that would render them absolutely invincible.
Following that, he joined up with his grandson, Jotaro, in order to put a stop to the revived Dio Brando. His role during these events was more or less a supporting part, though he did get a few moments to himself.
Kujo Jotaro.
Grandson of Joseph Joestar. A perpetually angry youth, Jotaro's first appearance has him sitting in a jail cell which he willingly put himself in to keep the people around him from getting hurt by his Stand, Star Platinum, which he had no clue about and had minimal control over. After Joseph assures him he's not cursed and the Stand is under his control, he is recruited by his grandfather and Muhammad Avdol to search out and kill Dio Brando before the vampire takes over the world. And in case you were wondering, he and the entire part 3 arc is what people normally think about the series.
Higashikata Josuke.
The illegitimate son of Joseph Joestar. Josuke is known for his unique hairstyle and fierce temper regarding said hair. His adventures started when Jotaro tracked him down and informed him that he was Joseph's son and warned him about the deadly Stand user Angelo, who killed his grandfather shortly after the start of the story. Following that, he is pulled into the search for the serial killer that threatens Morioh-cho.
His Stand is Crazy Diamond, which allows Josuke to repair objects as he sees fit. This can be applied in several ways, including healing wounds, breaking two objects and combining them into one, and reducing something to its original components.
Note : The second kanji in his first name, can also be read as "jo," as well as that his proper name would be Josuke Joestar.
Waiting for the Part IV Anime.
I think the Kujo Jotaro is pretty cool.