Mireille was born to two impoverished gambling addicts. She quickly bonded with her older brother, Lysander Halifax, who looked after her due to their parents' poor treatment of them.
During Hogwarts, the two did not fare much better. They were often bullied. As Mireille's body developed, she attracted several suitors who often anonymously left her roses. None admitted publicly to liking her because they did not want to damage their reputations.
Lysander was diagnosed with an extremely rare magical blood disease. While there was a cure, the family could not afford the potion. Lysander's health slowly declined and it was clear that he would not live for much longer. While Mireille attempted to comfort and take care of him, she continued to live her own life. She befriended other students and became more involved at Hogwarts. Lysander became bitter as he believed that Mireille had a bright future ahead of her.
Mireille tried to find ways to save her brother. While at first Lysander only wanted to be cured of his disease, he soon became obsessed with living forever. He convinced Mireille that they needed to become immortal together. They became entirely focused on their quest, and eventually found out about Horcruxes.
Both were initially against it due to the need to kill someone in order to create a Horcrux. However, Lysander's opinion changed as he became more desperate. Their parents finally won a decent sum of money- assumed to be enough to buy the potion. Lysander witnessed his parents enter a casino and became filled with rage as he believed that they were going to gamble the money away rather than save him. He abducted his parents and took them to catacombs within Hogwarts.
Meanwhile, Mireille was given a beautiful golden locket with a rose engraved on it by another suitor. She heard about what had happened and brought it to Lysander in the hopes that they could sell it and buy the potion. He had become so bitter and obsessed with immortality that he refused. He and Mireille fought and one of their parents was killed in the crossfire.