Millie is the daughter of Zankan, a skilled mechanic. Her mother died during a raid by the Inspection Force, which enforced a technology prohibition called the Technos Taboo; a stray bullet from a shootout between soldiers and a panicked customer struck and killed her, leaving Zankan to raise Millie alone.

She traveled with her widowed father as he operated a large, boxy armored crane and ran his mechanical business, assisting him in his work. Throughout this period, she developed a cheerful personality and formed a strong, sister-like bond with a jack-of-all-trades named Honoka, whom she affectionately calls Hono-chan.

After her father was killed by an autonomous enforcement unit known as Blue Breaker for repeated violations of the Technos Taboo, Millie was orphaned. She then began traveling with Honoka, who aimed to help her cope with the loss.

She eventually settled in a location called Emporium, attending school and living under the care of her paternal aunt, Esmel, who became her legal guardian. Despite this stable arrangement, she maintained her connection to Honoka's travels and her own mechanical interests. She received basic firearms training and briefly possessed a handgun. Her role within the group and her continued training suggest she is learning mechanics as an apprentice, following in her father's footsteps.