Mille is a Rider from Hakum Village, a companion to Lute, Cheval, Lilia, and Hyoro. She graduated from Hakum Village's Rider School alongside her classmates, receiving an Aptonoth as her first monstie. Initially serious and dedicated with a direct attitude, her personality shifts towards radicalism after joining Cheval's quest to destroy monsters infected by the Black Blight. Feeling used as a tool by Cheval prompts another shift, leading to a notably more cheerful demeanor by the end of the first season. She shared a close relationship with Lilia and expressed sadness when Lilia chose a path other than becoming a Rider. The Nargacuga incident in Hakum Village motivates her to join the fight against the Black Blight with her friends.

Mille possesses blue eyes and purple hair. Her first season attire consists of a pink dress, navy blue pants, sandals, and a scale-like hair clip. By the second season, her hair is shoulder-length; she wears a pink short-sleeve shirt layered over a darker pink tank top, light brown boots, and bracers. She initially wields a Hunter's Dagger given by Hakum Village's chief. In the second season, she upgrades to Regios Amor armor and replaces her dagger with a Cheda Blade Great Sword to battle the Ebon Riders.

Her monsties evolve throughout the series. Starting with an Aptonoth, she later obtains a Brute Tigrex capable of the Kinship Skill Rock Smash during her time with Cheval. In the second season, she switches to a Seregios named Legion, which undergoes the Rite of Channeling to gain fire abilities. When riding Legion, she uses armor and weapons crafted from Seregios materials.

Mille was the only female Rider in Hakum Village's Rider School and often chides Hyoro for his lack of confidence. She appears as a selectable skin for the protagonist in the game's 1.1 update. As an anime-original character not present in the original game, she is considered a Canon Foreigner.