Millis Steel, daughter of master thief Argus Steel, aids her father in pursuing the Mythical Pokémon Diancie to exploit its diamond-creating abilities. She has black hair, blue eyes, and glasses, wearing a black tank top partially covered by a green wrap with red stripes, secured by a white cord featuring a red ruby, along with blue jeans and green sneakers.

Initially posing as an ally, she rescues Diancie from Team Rocket in Avignon Town by unlocking a clock tower door. This assistance proves deceptive; she monitors Diancie through a spy feed from her father's Honedge. Her true goal is ensuring Diancie reaches Xerneas in Allearth Forest to gain stable diamond-creating power for capture. Millis taunts rival thieves Merilyn and Riot to distract them, enabling Ash and his friends to escape with Diancie. After Diancie bathes in Xerneas's Fairy Aura, Millis discards her facade, declaring she aided only out of pity.

During the conflict over Diancie near Yveltal's cocoon, Millis battles alongside Argus against Merilyn and Riot. Their struggle awakens Yveltal, triggering its rampage. Millis and Argus narrowly evade Yveltal's Oblivion Wing with Chesnaught's Spiky Shield protection but are petrified when their airship attacks Yveltal. Xerneas revives them using Geomancy, ending their petrification. Following these events, Millis and Argus abandon thievery to open a chocolate shop, as depicted in the movie's credits. Her Pokémon partner Chesnaught assists in combat and protection throughout these ordeals.