Millie

Millie is one of the main villain protagonists of "Helluva Boss", a spin-off series set in the same universe as Hazbin Hotel.

History
Millie is the loving wife of Moxxie and a fellow member of I.M.P, she has a sweet and caring personality but is also a ruthless and sadistic killer who enjoys her occupation as a killer for hire, she also somewhat enjoys being part of Blitzo's "family" and tries to bring peace between him and Moxxie (though it is also suggested she actually enjoys encouraging Blitzo's behavior).

Millie is seen to accompany Moxxie wherever he goes, she will defend him against any attack (be it Loone or even an insulting child) - unlike Moxxie she is much more willing to just go along with anything Blitzo wants and is most happy when out in the field committing mass murder.

Trivia

 * Millie is likely a true resident of Hell, which would make her a true demon rather than a sinner (such as most of the cast of Hazbin Hotel).
 * due to being imps Moxxie, Millie and Blitzo are seen as the lowest of Hell's citizenry, which may further explain their drive for fame and wealth.