Diavolo is the main villain from the fifth arc of JoJo's Bizarre Adventure, Vento Aureo. For the majority of the arc he acts as a unseen mafia boss, who hides in the shadows and seeks to erase his past.
Most of Diavolo's past is completely unknown, mostly because he got rid of everything that could lead to his identity. He was born in prison, and his mother gave him to a priest to raise him. When he was around his 15, the village where he lived burned completely, and he was supposedly dead in the incident (it is implied he caused the fire). It is also know that, before becoming a gangster, he discovered The Arrows, and sold five of the to Enya Geil in Egypt for a large sum of money and kept one arrow with himself, using the money later to raise in the ranks. He soon became the boss of the Passione gang, and started to organize the group into a large mafia with a huge influence. He also hid his identity and cleaned his past, so it would be easier for him to command the group. The arrow gave him his stand, King Crimson, which helped him with his obsession to erase his past. He later gave his arrow to his underling Polpo, in order to make everyone in the gang into stand users.
A Frenchman called Jean Pierre Polnareff was tracking the origins of the arrows, which led him to Italy and Diavolo. However, the mafia had a formidable control over everything, thus leaving Polnareff completely alone, all by himself in enemy territory. He confronted Diavolo, who beat him up severely, leaving him almost dead. He miraculously survived, and decided to hide until he could find someone trying to discover the boss's identity.
Much time later, Diavolo discovered that he had a daughter. He then sent the group of the recently promoted operative Bruno Buccellati to escort her to him, never revealing his true intentions, and always contacting then through a trusted agent, who also never knew his identity. Diavolo knew that there were members in the gang unsatisfied with being ruled by an unknown boss, and thus were trying to uncover his identity, so he decided he would kill his daughter by himself, so nobody would get anything from her. However, he never knew that Bruno was also trying to uncover his identity, and the two confronted each other, with Diavolo always hiding. He almost killed Bruno, who was saved by the new member Giorno Giovanna, who entered the gang already with the goal of overthrowing the boss in mind. Without much options, Diavolo sent his assassins after Bruno's group, but they were all defeated.
Diavolo then decided to take action, and sent his alter-ego Vinnegar Doppio after them. Both Diavolo and Doppio were the same person in the same body, but Diavolo uses his alter-ego to hide. Each personality had different physical appearances, and Diavolo could talk in Doppio's mind, as he always thought that the boss was talking to him by phone calls from strange things like bananas and frogs. With this, he managed to kill one of Bruno's partners, as well as another member who was after him. But he wasn't expecting that Polnareff was still alive, and also trying to contact Bruno's group. Diavolo ambushed them by using Doppio's disguise. He managed to kill Polnareff, but he was holding another arrow, which pierced through Polnareff's stand, turning it into a requiem stand, which immediately switched everyone's spirits. Diavolo's spirit manged to enter in a body along with his daughter, and while Bruno's group was trying to figure a way to retrieve the arrow from Requiem, Diavolo hid himself among them, trying to approach Requiem and get the arrow by himself, but Giorno noticed, forcing him to run after Requiem. He managed to defeat Requiem, but it was Giorno's stand who was pierced by the arrow. Giorno's new stand nullified Diavolo's power and punched him severely. Due to Giorno's new power, Diavolo is now infinitely re-experiencing his own death, over and over.
Diavolo's stand, King Crimson, has the power to “erase” time. To simplify it, he activates his power, which lasts for ten seconds. During this time, Diavolo is able to see every movement like a sketch. After his spell ends, anything that happened during the time his power was active will be simple erased out of existence, like it never happened. This not only will leave his enemy disoriented (thinking like it was teleported), but also allow Diavolo to predict his opponent's moves and not only dodge it but also position himself to counter the attack.
He is over-obsessed with erasing his past and identity at all costs, but not much else is known about him.