Argus Steel is the main antagonist of Diancie and the Cocoon of Destruction. He is a master thief known even to Team Rocket.
He is the father of Millis Steel, and the rival of Merilyn and Riot. Together with his daughter, he plans to awaken Diancie's power to create diamonds before stealing the Pokémon and fortune for himself.

(Source: Bulbapedia)