Millis Steel is one of the antagonists of Diancie and the Cocoon of Destruction. She is the daughter of master thief Argus Steel and works with her father to obtain Diancie's diamonds.

Millis debuted at the end of Diancie — Princess of the Diamond Domain, monitoring Diancie via a secret spy feed recorded by her father's Honedge.
In Diancie and the Cocoon of Destruction, Millis opened a door to the clock tower in Avignon Town to help Diancie escape from Team Rocket. She returned to Argus's airship to inform him about Diancie's progress.
Later, when Merilyn and Riot attacked Ash and his friends, Millis taunted them both to allow the group time to escape. When Diancie was bathed by Xerneas's Fairy Aura in Allearth Forest, Millis and Argus arrived. Millis revealed that she only helped Diancie because she was feeling sorry for her. Merilyn and Riot also arrived and the thieves fought over Diancie. They took their fighting to an underground pool where Yveltal slept in cocoon form, where the fighting eventually awakened the Destruction Pokémon.
During Yveltal's rampage, Millis, Argus, and Aegislash narrowly survived Yveltal's Oblivion Wing, but their Pokémon didn't, though Chesnaught protected them with Spiky Shield. When they returned to the airship to fire missiles at Yveltal, they were petrified by its Oblivion Wing. Millis, Argus, and their Pokémon were revived later on by Xerneas's Geomancy.
After these events, Millis and her father gave up their criminal life and opened a chocolate shop.

(Source: Bulbapedia)