Gren is a Cyborg. A big different. I can't say I know how the technology is done, but in my head cannon Gren uses his aura to power his body's basic function, like moving. He has a backup when his aura is down to zero, but that's only so he can escape/recover. Gren's fusion core powers everything else, like his Scroll, inner computer, rocket boosters, and programming his rockets functions.