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. In "Murder Family" she appeared to be very unsympathetic about killing a family, only caring if it was what the client wanted, showcasing her dedication to her work.