Millis Steel, daughter of master thief Argus Steel, emerges as a secondary antagonist with striking black hair, blue eyes, and glasses. Her wardrobe—a black midriff-baring tank top, a green towel-like wrap adorned with red stripes fastened by a white cord and ruby clasp, blue jeans, and green sneakers—reflects her sharp, practical demeanor.

Posing initially as an ally, she aids Diancie’s escape from Team Rocket by unlocking a clock tower, a maneuver later exposed as a strategic ruse to track Diancie and exploit her diamond-generating powers. While driven by her father’s ambition to seize Diancie for personal gain, Millis intermittently voices irritation at Argus’s compulsive chocolate indulgence, hinting at a flicker of familial friction.

Her schemes intensify when she clashes with rival thieves Merilyn and Riot, their skirmish accidentally rousing Yveltal from its dormant state. Amid the chaos, her Chesnaught shields her from Yveltal’s lethal assaults, though she succumbs to petrification via Oblivion Wing. Revived by Xerneas’s Geomancy, she abandons thievery post-crisis, joining Argus in managing a chocolate shop, as glimpsed in the narrative’s closing scenes.

Her arc pivots from cunning manipulation to redemption, catalyzed by Yveltal’s devastation and Xerneas’s restorative influence. Once fixated on exploiting Diancie, she ultimately renounces her criminal identity, her journey framing destruction and renewal as twin forces reshaping her path.