The Resistance is a group that appears in Sonic X. It was a small group of human soldiers who fought the Metarex in order to restore the glory of their now-destroyed planet, Cascade. Molly and Leon are the only named members of the Resistance.
Eventually the rest of the Resistance were rounded up by the Metarex. They were all consumed by trees which grew around them, killing them.
Since the Resistance fighters were the only survivors of Cascade after the Metarex attacked, their death leaves the Cascadians virtually extinct.