It is clear that Millie loves both her mother and father, as she happily ran up and hugged them while visiting for the Harvest Moon Festival. However, she can get a bit aggravated in regards to their behavior towards Moxxie and is not afraid to stand up to them, along with finding their scolding of her overly violent tendencies to be unfair.
It is believed that Millie has a typical sibling rivalry with her sister. She is a bit miffed over how Sallie May claims to be better at hiding bodies and therefore able to compete in the festival's activities while she is barred from doing so.
Moxxie is Millie's husband and co-worker. They have a positive relationship and both adore each other. Moxxie even dedicates a song called "Oh Millie" to her, which can be seen at the end of the pilot. The two are very affectionate and passionate with each other, and often defend each other from accusations.
Millie is also very protective of Moxxie. In Spring Broken, when the Catfish Monster tried to eat a drunk Moxxie, Millie rushed to his rescue and single handled killed the monster. In The Harvest Moon Festival (episode), when she saw that Striker was trying to kill Moxxie, Millie went mad with rage and stabbed Striker multiplied times in the back.
Blitzo is Millie's boss. He is the founder and manager of Immediate Murder Professionals. Millie works as his bruiser alongside her husband, Moxxie, who is his weapons specialist. Millie seems to not mind Blitzo's invasive actions and appears to like her boss. She does sometimes tease him herself such as when she told her parents he was Stolas's "boyfriend".
Loona and Millie don't seem to get along. As in the pilot, Millie can be seen flipping Loona off after Loona insults Moxxie. In later episodes Millie usually refers to Loona as "our Hellhound" or "Blitzo's Hellhound", which irritates Loona.
Millie did not seem to mind being insulted by him but she drew the line when Eddie starts degrading Moxxie.
Initially, Millie had a cordial relationship with Striker who was working on her family's farm as a ranch hand. However, this becomes antagonistic when she finds him trying to kill Moxxie and stabs Striker repeatedly in the back. She refers to him afterwards as a "brow nosing cock-sucker", and she also vocalizes the fact that he was an assassin to her parents since they hired him in the first place.