Tails Nine is an alternate version of Tails that was created when Sonic broke the Paradox Prism with his battle with Eggman and lives in an alternate reality where the hedgehog never existed, resulting in secluding himself from society with only giving himself seven robotic tails and inventing machinery, to protect himself from many years of torment.
While initially depicted an anti-heroic ally, Nine feels betrayed when he realized that Sonic didn’t want the same thing, This results in him deciding he cannot trust Sonic and stealing the Paradox Prism to create a "perfect world" in the Grim, possibly to cause more problems for Sonic to face.