Mag was born blind on 07/05/17. She was Marni's best friend, so Rotti Largo decided to operate on her. On 03/21/40 she got her new eyes, but she didn´t realize that she had signed the contract in blood. Now she had to serve Rotti Largo, who used her voice as advertising for GeneCo.
Mag was Shilo's Godmother, but was, up until meeting Shilo via Rotti Largo, unaware that Shilo was still alive; Nathan had told her that Shilo died with Marni. After the show, Mag goes to Shilo's house and urges her not to follow the same path she did by signing a contract with GeneCo ("Chase the Morning").
Nathan returns home to find Mag in the house. Shilo attempts to stop him from throwing her out, knowing about the consequences of Mag breaking her contract. Mag assures Shilo that she'll be fine while simultaneously trying to convince Nathan that Shilo shouldn't be locked up in the house ("Everyone's a Composer"). Eventually, Mag is thrown out.Mag sings her last song ("Chromaggia") at the "Genetic Opera" powerer by GeneCo. At the end of the song, she gouges out her own eyes. Rotti Largo cuts the rope that's suspending her in the air, and she falls onto a spiked prop fence, dead.
Mag is generally depicted as being calm, cool and collected. In the face of Amber's hostility, she walks away and keeps her head. She displays some surprise, and maybe a bit of fear, at Rotti's line: "Technically you belong to GeneCo", at the ribbon-cutting ceremony.
Mag also seems to be the most mature character of the cast. To some extent, she seems to shelter herself from interaction with the other characters except when necessary. She only breaks this when she actively seeks Shilo before her final opera. It is unknown if this sheltering is a result of the death of her best friend, Marni.
Mag is established as having been best friends with Marni, though how long that friendship existed for is unclear. Before her death, Marni had asked Mag to be godmother to Shilo.
Mag is Shilo's godmother, though she was unable to play that role in her life after Marni's death (Nathan told Mag that Shilo had died as well). Though their interaction is brief, Mag shows great concern for Shilo's future, urging her to avoid signing on with GeneCo and Rotti Largo, knowing that Shilo will probably be shackled to the company the same way that she was.
Little is known about any friendship that Mag and Nathan may have had. They knew each other, and they greeted each other using their first names at the beginning of 'Everyone's a Composer', implying that they were, at one time, friendly. Mag was upset to learn that Nathan had lied to her and concealed Shilo for seventeen years; the pair argue over this, and eventually Nathan throws her out. However, Nathan still refused to reposess Mag's eyes, and seems to be shaken upon seeing Mag's dead body on the stage at the opera.
The only interaction seen between Mag and Rotti is at the ribbon-cutting ceremony, and there is never any direct information on their relationship. However, given that Mag was bound to GeneCo at the threat of losing her eyes, it is generally assumed that she despised Rotti- for this, and for his obvious attempt to bring Shilo over to GeneCo in the same way.
Of all of the characters in the movie, Amber is the one who acts the most hostile towards Mag. Amber wants to be the spokeswoman for GeneCo, the one doing the singing, and she sees Mag as the only thing in her way. Notably, Mag does not return any of this hostility, brushing Amber's attack on her off at the Italian Renaissance Fair.
"Tao of Mag"
"Chase the Morning"
"At the Opera Tonight"