Mielle is backed by Princess Isis, Oscars sister. She has power over the Prince since the two of them are going to get married. Oscar falls for Aria, which pisses off Isis. So she joins Mielle in trying to get rid of Aria.
Mielle is a bitch. I'd argue even worse than Aria.
She tries poisoning Aria. This results in her personal maid that raised her dying.
She pushes their father down a flight of stairs, paralyzing him for life, and tries to blame it on Aria.
She runs way to a neighboring kingdom with Princess Isis after their plans fail time and time again.
She immediately ditches Oscar when she thinks the King of the neighboring kingdom is interested in her.
She and Isis try to start a rebellion, but are IMMEDIATELY foiled by Aria and the Prince, leading to Isis and all the corrupt nobles getting executed.
Aria who has not revealed her true colors to Mielle yet, "Saves" Mielle from being executed, and imploys her as a maid. Having her taunted / abused by one of her close maids for a while. This arc was kind of hard to read for me. Not because I felt bat for Mielle, but because Aria pretending to treat Mielle nicely felt gross.
Aria suggests Mielle give the maid that has been harassing her a stomach ache / prank her. Aria tells her a good poison she could use, but to only use a little bit because more would be lethal. And well, as you can probably guess, she uses a lethal amount because she's a spiteful bitch.
Aria plans it so that Mielle's last ally / family member, her brother, is the one that drinks it. In the eyes of everyone else, it looks like she tried poisoning Aria, so not only did she kill her brother, but she's also sent to death.
Her head rolls.