- "That's my husband you're talking to!"
- ―Millie, to Eddie
Millie is one of the four main characters in Helluva Boss. She is the bruiser of Immediate Murder Professionals (I.M.P), a startup assassination business that carries out its services in the living world.
Millie is a short Imp with a long, demonic tail with white markings. She has eyes made up of yellow scleras and black pupils. She has long black eyelashes that extend beyond the sides of her hair.
She has two black horns with three thin white stripes and greyish-black hair with a white splotch in the center and bangs that cover half of her right horn. She has a beauty mark on her left cheek and a gap between her front teeth.
Her attire consists of black lipstick, a simple black choker, a cold-shoulder black crop top with gold buttons where the straps at the top meet the torso piece, torn black pants, fingerless black gloves, and black footwear that does not cover her hoof-like toes.
Millie also has a black tattoo in the shape of a heart on her right shoulder, and two white splotches on the insides and outsides of each of her elbows.
Millie is very lively and cheerful. Often with a smile on her face and rarely standing still, she is a clear extrovert and isn't afraid to speak her mind. Whether it's expressing her excitement or flipping off a coworker.
Also, as Millie is the bruiser of Blitzo's employees and seems to enjoy her job, it can be assumed she's tough and very violent as well. According to Moxxie, she is aggressive in the sack as well as her job.
- Viv has stated that she would change Millie's voice actor once the actual series premieres but would still keep her previous voice actor, Erica Lindbeck, to voice Loona. Later, a new voice actress, Jennifer "Jenny" Bingham (aka ThatJennyBee), was hired to voice Millie.
- There is a large photo of what could possibly be Moxxie and Millie's wedding photo in the meeting room. In it, Millie is wearing a long strapless black dress and has wild hair that goes down past her hips, while Moxxie is wearing a white suit. It appears Blitzo has drawn over the photo in red marker to draw himself hugging them.
- This same photo seems to be in their living room although you can only see the bottom half and it doesn't have Blitzo's graffiti.
- In Vivziepop's 2019 short Holidaze as an ornament on a Christmas Tree, there is a tree carved with initials "M+M →B←" in a heart that alludes to her and Moxxie (and by extension Blitzo).
|Main characters||Hazbin Hotel||Angel Dust • Alastor • Charlie • Husk • Niffty • Vaggie|
|Helluva Boss||Blitzo • Millie • Moxxie • Loona • Stolas|
|Overlords||Alastor • Lilith • Lucifer • Rosie • Valentino • Velvet • Vox|
|Supporting characters||Baxter • Cherri Bomb • Crymini • Egg Bois • Exterminators • Helsa • Katie Killjoy • Mimzy • Razzle and Dazzle • Seviathan von Eldritch • Sir Pentious • Tom Trench • Travis|
|Others||Arackniss • Barbie Wire • Collin • Eddie • Fat Nuggets • Henroin • Loo-Loo Land Mascot • Molly • Octavia • Roo • Robo Fizz • Tilla • Villa|