@CyraniaWell shoot, then I'd be happy to toss in either one. Some rejiggering of the backstory for both, certainly (which wouldn't be too difficult - Replace a few secret organizations here and there and you've got all kinds of backstory to work with). So which would work better with the group?
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So for Riot, he was part of a shadow government program to create enhanced law enforcement personnel specifically to deal with super-powered threats. The program's secondary objective, however, was to remove free will from the successful volunteers, to create
unquestioning enforcers for the US government. Robert Carter (Riot) discovered the plot as the first batch of "Protectors" were successful, a batch of seven. He showed the other volunteers the evidence of what was happening, and started a revolt, though only two others believed him. Four directly sided with the government, and there was a skirmish, in which he believes he was the only survivor.
He was able to destroy the facility, getting away with a truck full of gear that he's been using to subtlely fight back against corruption and crime in the city.
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Alter was a bit of a different story, as he was actually from a decent middle-class family, who was killed in a freak airline accident on the way back from Paris to the United States. Young Paul Phillips, or "Polo", was always a smart kid, always becoming good at whatever he set his mind to, at least until he felt that he'd mastered it and moved on. When his family was killed, he received a sizable inheritance from his parents' life insurance policies, but still finished his degree in electronics and bioengineering, though it was more for the sake of his family than for himself.
Later, he was approached by a shadowy organization and persuaded through his grief to work on something that "really mattered." Turns out, he was working to put super-powered people to use doing good, even if that was against their will. He utilized his abilities as a biotech to create numerous types of technology to help their missions, while still kind of questioning whether what he was doing was right. These included a Kill-Switch Protocol to keep the more dangerous "powers" in check, as well as a hard-light obstacle training simulator, or "HOT Room" for power training.
Eventually, he was shocked into a face-to-face confrontation with the actual evil that he was helping to perpetuate, and so he worked with several of the other powers to bring the organization to its knees, escaping into relative obscurity in the panic, and working to help others mostly behind the scenes.
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Which of these would help to "fill the gaps" better?