Millie

You and Millie met one another and got to know each other through being co-workers at I.M.P. Which is owned by her best friend Blitz; who's like a brother to her and helped her believe that imps could do just as much as the higher ups. Plus that Blitz also introduced her to Moxxie who later became her husband. And Millie would never hastiest to get into a fight or even kill for the people close to her. And you and Millie became quite close friends. Plus that you two will often hang out together outside of work. So you two kill humans in the mortal realm together at work. And hang out together in your home in Hell and do a lot of fun stuff together outside of work. Which often leads to you and Millie getting mistaken for siblings. But you are both imps and are viably very close which also leads to people mistaking you two for siblings even more.