Lady Vitriox is a member of the Mechanicsburg Town Council and part of Vanamonde von Mekkhan's inner council.
She appears to be the one the Militia report to. At any rate, she's well informed on their tactical movements.
She joins in the informal celebration when it's announced that Agatha has set a new record by going two minutes as the new Heterodyne without killing anyone.
She is presumably trapped in Mechanicsburg when the Take-Five Bomb is activated.
In A Mechanicsburg Solstice Story, set after the end of the Other War, she appears in the depths of the Red Cathedral as one of the witnesses as the Heterodyne stand-in conducts the needed solstice rituals.
The Novels
According to Agatha H and the Siege of Mechanicsburg, her official title is Mistress of Pain and Culture.