The Strongest Hero with a Negative Destructive Power

It's been decades since monsters flooded the world. Our protagonist enrolled in an academy that trains capable fighters, but he always came dead last in ability tests. It was bad enough that his powers were weak, but he also dragged down his teammates by giving them debuffs that weakened their abilities. He was mocked as "useless" and "dead weight."

One day, while out on an exploration mission, he got sucked into some kind of dimensional rift and vanished without a trace. That's where he met creatures from another world who finally showed him what his abilities were really worth. See, magic is supposed to work by pushing your power outward to attack, but his gift worked the complete opposite way — instead of "output," he had "input." His real strength was absorbing outside forces, something called a "Reverse Magical Circuit."

All those times he'd been weakening his teammates? He'd been accidentally draining their power without even knowing it. Once he figured out what he could really do, everything changed. He started absorbing monster attacks and canceling them out completely, drawing in external energy to make himself stronger, and finally began his journey to become the most powerful hero of all.