Argus Steel shows up as the main villain in Diancie and the Cocoon of Destruction. He's a legendary-level thief—seriously skilled enough that even Team Rocket knows his reputation.
He's Millis Steel's father, and he's got a rivalry going with both Merilyn and Riot. His whole scheme involves teaming up with his daughter to unlock Diancie's diamond-creation abilities, then grabbing the Pokémon and whatever fortune comes with it for themselves. It's basically a father-daughter heist with a legendary Pokémon in the crosshairs.
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