Jasmine's Steelix is Jasmine's most powerful Pokémon that she initially disguised as its pre-evolution, an Onix.
Steelix is Jasmine's powerhouse Pokémon and she used it to stop Team Rocket with brute force. In her Gym battle with Ash Ketchum, Steelix easily defeated Ash's Pikachu with powerful Iron Tail and then battled Ash's Cyndaquil. Steelix used Sandstorm to block Cyndaquils Flamethrower, and used Dig within the Sandstorm. Eventually it smashed Cyndaquil with a devastating Iron Tail almost defeating it. However, Cyndaquil activated its Blaze ability (or it would have done if special abilities had been invented at the time) and blasted Steelix with the strongest Flamethrower it could muster, roasting Steelix within its own Sandstorm and defeating it.
Steelix also appeared in Four Roads Diverged in a Pokémon Port along with Jasmine, where it fought Flint's Infernape. It put up a good fight but lost quickly.