Millie is a young woman with light skin and eyes that are slanted downwards with long eyelashes. Her chin-length hair is styled into a bob with her fringe swept to the right of her face.
She is seen wearing the Berlint City Hall uniform with a vest over a long-sleeved shirt and a skirt. Her skirt is noticeably shorter than her co-workers, with thigh-high black socks. She also wears a small ribbon to the right side of her head and a pair of round earrings.
At the party, she wears a strapless dress with frills along the top and a belt fitted below the bust. She sports a pearl necklace with her round earrings and the ribbon on the right side of her head is thinner. Her high heels have flower decals along the back.
She's shown to be lively and mischievous, suggesting to Yor that they put snot in the Chief's coffee as a joke and most often seen smiling or giggling. Even though she later breaks up with her boyfriend, she doesn't seem bothered and remains cheerful, even asking Yor to introduce her to Yuri.
She has a boyfriend who she wears short skirts for, stating that he loves when she wears them. They break up at some point before the SSS arrest Jim Hayward at Berlint City Hall.
During their break, she gossips with Camilla and Sharon about the recent robbery at City Hall but moves on to complain about their section chief ogling her that morning, which creeps her out more than the robbery.
Millie retells about Jim Hayward being arrested by the SSS that morning to her co-workers. She complains loudly, wishing it was their section chief who was arrested instead just as the chief walks by. When Camilla asks about Yor not telling Yuri about her marriage, Millie wonders why Yor's marriage is such a mystery. She asks Yor if her brother is attractive and to be introduced to him, revealing she broke up with her boyfriend the other day when Sharon reminds her. When seeing the picture of Yor and Yuri, she describes him as cute excitedly but is put off when she realizes Yuri has a sister complex.
Millie invites Camilla for a drink but is rejected as Camilla is going to teach Yor cooking that night. Three days earlier, when Yor requests their help in learning how to cook, she offers to teach Yor how to use a can opener.