Mydia is a viera. She belongs to the feol viera, a group of viera descended from the aegyl god Feolthanos and his viera wife. The feol viera were considered abominations by their viera kin and retreated to the Roda Volcano, where they lived in the Feol Warren.
In her youth, Mydia would argue with her mother about the fate of her people - to be hidden in the Feol Warren, away from the rest of Ivalice. She left the Feol Warren upon coming of age.
She eventually traveled to Rabanastre, the capital of Dalmasca, and met a hume named Velis. The two became friends and eventually fell in love. However, Velis was called away to Nalbina to defend the fortress from the Archadian Empire. He was later slain at Nalbina.
Remembering the stories from her youth about the Eternal, a source of eternal life she thought could revive Velis, Mydia wandered Ivalice, getting clues about the islands of legend. She eventually found the Cache of Glabados, where she acquired a piece of auracite and the airship Galbana. Using this ship, she flew off to Lemurés.
In Lemurés, Mydia encountered Feolthanos. He detected her anima, seeking to control it. This was easy due to her kinship with him, coupled with her possession of auracite and her rage over the loss of Velis. Feolthanos stole Mydia's will and transformed her into the Judge of Wings. Mydia is called this due to resembling an Archadian Judge.
Collecting the Auraliths
Eventually, Vaan and his party meet the Judge of Wings on Tormelados, Isle of the Megalith. She had drained the auralith of its energy, and upon encountering the party, uses her powers to distort their feelings and knock them unconscious. She shows them a flashback of an encounter with Balthier and Fran. After knocking out Fran, Balthier tries to stop the Judge by destroying his own auracite and threatening her with his gun. The Judge responds by summoning Shiva, who knocks out Balthier.Back in the present, the Judge fights the party. She then leaves, leaving them to fight Shiva, and destroys the auralith. The party later finds her in Yapih Caverns, and fights her again. But she uses Bahamut to overpower them. Bahamut shatters the Skysea to several pieces.
The party later encounters an illusion of Velis, who asks for the whereabouts of Mydia. When the Judge attacks the party, they realize from Velis' reaction that the Judge is actually Mydia.
The Judge shows the party another flashback. She was at the Yapih Caverns in order to summon Velis back from the dead, using Feolthanos' power. However, the form she brought back was unresponsive to her, and Velis was spirited away in a flash of light, landing on Tswarra. Back at Tomaj's Camp, the Judge captures Velis is a globe of light and transforms him into the Yarhi Odin. After a while, she comes to the conclusion the Velis she has resurrected is not the true Velis she loved, so she leaves the island, and the party is forced to defeat and kill him.
Back in Ivalice, the party encounters the Judge in the Paramina Rift. Basch tries to defeat the Judge, but she teleports away. The party searches for clues around Ivalice on Mydia's location, which leads them to the Feol Warren in the Roda Volcano.
Tragedy was already apparent at the Feol Warren, where the Judge is going on a genocidal rampage against the feol viera. She claims that this is the only way to end their suffering. The party tries to stop her, but she summons Chaos to help her fight them. After Chaos is defeated, she throws her auracite in the lava, summoning an army of wyvern.
The party defeats the wyvern, and notice that the Judge's armor disappeared, revealing herself to be Mydia. Mydia reveals that she was possessed by Feolthanos, who took advantage of her fierce anger. She asks the party to slay Feolthanos, and then keels over and dies.
Later, Feolthanos would use Mydia's anima to create an illusion of the Judge of Wings, but this is defeated as well.