Giacomo Fortunato is a son of Don Vincente Fortunato and a member of organized crime. When Kingpin / Wilson Fisk lost control of the Underworld in New York City, Vincente moved to fill the power void. But to do so the Fortunatos acted as representatives of HYDRA. Giacomo protested this alliance and the extreme methods used by his father. Vincente was furious that his own son would dare undermine him and ordered the assassination of Giacomo. In an attempt against Giacomo's life, Ben Reilly was almost gunned down instead. Jimmy-6 saved the life of the "innocent" bystander, holding to his own honor code. Reilly returned the favor by protecting Jimmy's life as Spider-Man. Reilly and the junior Fortunato formed a friendship of shorts.
Later Giacomo learned of Hammerhead leading a coalition of gangs against Don Fortunato and an assassination plot against the Don. He tried to warn his father, only to learn his father was more bloodthirsty than previously thought. The Don had abducted civilian hostages from all the areas "protected" from the rival gang lords. At the first sign of the hit against him, all the hostages would be executed. Giacomo was sufficiently enraged to pull a gun on his father while Vincente calmly disowned him.
Giacomo allied with Spider-Man (Reilly) and Daredevil to rescue the hostages. When Reilly was killed by the Green Goblin / Norman Osborn, Jimmy vowed to avenge him. But his friendship would not end here. A wounded Jimmy would seek help from Peter Parker, thinking he was a "cousin" of Reilly, and would help protect Peter and Mary Jane from Goblin-employed assassins. He also helped Spider-Man against bounty-hunters employed by Osborn. He could empathize as there was a price on his own head, courtesy of the Don.
While again facing assassins, Giacomo received help from Punisher / Castle and Shotgun. Mostly because both vigilantes thought the assassins were more dangerous than Jimmy. When Kingpin returned to reclaim his position, Vincente was violently deposed and hospitalized. Giacomo took over as head of the Fortunato family. He initially attempted to peacefully reconcile himself with Kingpin. A gunfight between their subordinates ended the negotiations and started the gang war Jimmy was trying to prevent. Jimmy himself, his wife and children were targeted by the Enforcers (Fancy Dan, Montana, Ox). Spidey helped protect the non-combatants.
The subsequent fate of Jimmy is unknown. Vincente Fortunato has been depicted as having regained control of the Fortunato crime family and spoken in favorable terms concerning Giacomo. They might have reconciled. A brother or half-brother of Jimmy briefly served as a host to Venom.
Jimmy looks like a younger version of the Kingpin, large and seemingly obese. But all his "fat" is actually muscle. He has peak human strength, above average agility and appears remarkably resistant to bullet and knife wounds. He is a fierce hand-to-hand combatant. He is a skilled strategist and has received training in electrical engineering. He is also proficient with various conventional weaponry. Besides the typical mob training, Giacomo has had military training. He reportedly has served "two tours of duty" with the United States Army. While fairly ruthless in dealing with opponents, Giacomo strives to keep civilians out of harm's way and collateral damage to a minimum.