Millis Steel is a teenage-girl with black hair, blue eyes, and wears a pair of glasses. She wears a black tank top that slightly reveals her midriff underneath a green towel-like wrap around with red stripes, which is connected to her tank top by a white cord with a red ruby on it, as well as an oval-shaped piece of cloth around her neck. She also wears a normal pair of blue jeans and green sneakers.
As the daughter of master thief Argus Steel, she has taken pride in assisting him. When Diancie was unable to make diamonds properly, Millis took on the facade of having Diancie get to safety. However, after Diancie was bathed by Xerneas's Fairy Aura, Millis's facade disappeared and continued her goal of getting Diancie. She still retained her antagonistic personality during Yveltal's onslaught despite being saved by Ash and his friends.
Millis would often tease Argus about his chocolate-eating habits.
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 unde`rground 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.