His death and forced return marked the change in Avengers history. Introduced in Deadly Hands of Kung Fu #22 (1976) by Bill Mantlo and Keith Giffen, Jonathon Hart, nicknamed Jack, is the son of a brilliant scientist and his Contraxian mother. His father was researching Zero Fluid, a liquid that could be used as an energy source. His mother later faked her death, with both his father and Jack not knowing her true origins. His father perfect the Zero Fluid and was going to donate it to the world to give them free energy but was killed by the Corporation, who wanted to use the formula for profit. During the fight, the Fluid covered Jack and caused his body to mutate into his current form. Jack swore revenge against the Corporation and fashioned a costume identity as Jack of Hearts.

He would attack the Corporation based in a blind path of revenge and would fight other heroes like the Hulk and White Tiger (Hector). They, however, would help him take down the Corporation with the aid of Iron Fist and Shang-Chi. He would later help Iron Man prevent a takeover of his company from Morcedi Midas. As his power became harder to control, he turned himself over to SHIELD but was kidnapped by the Contraxian race, who needed his hand in helping to save their sun. He was eventually captured by the Strange before being rescued by Quasar. He would return to Earth but a battle with the Presence saw his armor cracking and nearly exploded. He was rescued by Moondragon and began stay in Hart Residence, but his power became even more uncontrollable, and he journeyed into space to seek aid from the Silver Surfer.

The Surfer would help Jack by taking him to an alien armorer to repair his armor and he would aid Surfer in their battle against Tyrant. Weaken from his battle, he was nursed to health by Ganymede who would become his lover, but they broke up soon after. He would later be discovered badly hurt by Quasar, who summoned a team of Avengers to fight the Infinites. He would remain with this team and aided Earth in preventing the planet from becoming a Prison colony by the Intergalactic Council. He would remain with the Avengers for several months and was eventually invited to join the team. While on the team, he would clash with Ant-man (Lang) a number of times. After several missions together however, when Cassie, Lang’s daughter, was kidnapped, he went to her rescue. But damage to his suit caused him to journey into space and explode dying in the process.

However, his body was forced to revive by Scarlet Witch and crashed into the Avengers Mansion. When Lang went to find out how he had returned, Jack exploded and killing Lang. This marked the beginning of the Avengers Disassembled arc. (Lang would eventually return with some time travel). He would eventually be revived by Project Pegasus, who had used the Zero Fluid to create zombie version of the Squadron Supreme, who he defeated. He would eventually find himself in She-Hulk apartment and after a few adventures, the pair became romantically involved. They continued to adventure together, even aiding the F4 in the Reckoning War Arc.