Reinrassic III made his first appearance in Alone Together. During a fight with him, Kevin Levin damaged the Teleporter Pod that Reinrassic attempted to use to escape, sending Echo Echo and himself to Turrawuste, which is used only as a relay station. Though they needed each other to survive, saving each other on multiple occasions, Reinrassic showed the typical Highbreed hatred of other species, having Ben to walk ten paces behind him and insulting him constantly. Reinrassic lost his hand saving Ben from a Rodilia Dentia, resulting in Swampfire reattaching it, changing Reinrassic's arm into a Methanosian arm. Eventually, they made it to the Teleporter Pod, but Reinrassic placed himself in exile in Turrawuste, believing himself infected by Ben Tennyson.
In War of the Worlds: Part 2, after Ben healed the genetic damage of all Highbreed by fusing them with DNA in the Codon Stream. Reinrassic was unaffected by this because of his Methanosian arm. He, despite his original self-imposed exile, realized that the Highbreed's views on other species were wrong and that being fused with another alien's DNA wasn't a curse, but rather allowed the Highbreed to survive. This caused the Highbreed Council to elect him the new Highbreed Supreme. He called off the invasion and shook Ben's hand as a show of respect.
In If All Else Fails, Reinrassic aided Ben to stop a Highbreed weapon left over from the war, which had been activated by a Highbreed who was unaware the war was over. He told the officer he is the Highbreed Supreme but the officer noticed his arm and didn't believe him. Reinrassic opened his palm to reveal the Imperial Seal, and the officer knelt before him, apologizing. He ordered the officer to shut the weapon down, but the officer revealed once it was activated, it cannot be stopped. However, Ben and Gwen came up with a plan to enter the weapon and deactivate it from the inside, which was successful. The Highbreed Officer took control of it, preventing it from destroying a Nuclear power plant. Reinrassic congratulated Ben for stopping the weapon, but Ben revealed the Highbreed officer's sacrifice. However, Reinrassic told Ben that the Highbreed who died was a soldier doing his duty and that there was no greater honor for a Highbreed. Gwen Tennyson asked if there are any more Highbreed weapons on Earth. Though he said there were none, he mentioned that he will check the records anyway. Ben then offered him a ride home.