"My dearest Haedrig, Do not feel despair, my love. You did everything you could. Our time together meant more than words can say, but in the end fate is a cruel mistress. Your strength is needed to end the horrors that beset this world. My final wish is that you find your path. Love always, Mira."— Mira's Letter to Haedrig(src)
Mira met Haedrig in a town on the outskirts of Kurast. She was locked in a cage, to be burnt as a "witch." In the middle of the night, Haedrig broke her out, and they escaped. Her captors didn't chase them far—Haedrig reflected that this was likely because they were pleased to see her go.
Soon after meeting, the pair passed near Caldeum. She wanted to stop and see the city, but Haedrig convinced her to travel on.
At some point, on Mira's urging, the couple headed west to Tristram. Haedrig would later wonder if Mira had known what her future held, and had willingly led herself to her doom, in order to get Haedrig to a place where he could aid the Nephalem.
When a hero/group of heroes came upon her, she was still human, and begged for her husband to save her. Shortly afterwards however, the plague set in, and she turned into a Risen Dead, leaving the Nephalem with no option but to put her down.
Mira appears in the Cellar of the Damned in the first act of Diablo III, during the quest A Shattered Crown. She battles the player as a Super Unique Wretched Mother with the title "Wife of Haedrig". In combat, she has Knockback affix, and spits empowered, charged up bolts dealing greater Poison damage. In return, she cannot vomit new Risen Dead.
A vision of her later appears in the game's fourth act in the quest Prime Evil where she asks the player, "How could you convince my own husband to kill me? I could have been saved, but you made him cut me down!". After that, she turns out to be the Aspect of Lies in disguise.