The mage girl seemed to have three personalities - one was collected/intellectual, one was rash/violent, and one was cutesy. When the rash/violent personality was the active one, even her eyes turned red. I didn't notice if the cutesy personality had different eyes, but you could definitely hear the distinction between all three of the voices.
Is there a plot-related explanation for this, or is it just part of her character?
Female Mage was an "artificial" hero created by a sect of the Holy Church, called the Magic Association. She as taken in a a young age and isolated from the world... trained and such by the Association as she was to be a substitute for the Hero, in case he was defeated (in which cae the Association would have much to gain in terms of political influence).
"Silver" 銀 (ぎん, Gin) is the childish, sleepy personality is the primary personality, she is the resulting "masterpiece" of all the so-called "training."
In the Gaiden/Side-story まどろみの女魔法使い (Madoromi no Onna Mahoutsukai, the Sleepy Female Mage) she briefly describes her other "sisters," as she calls them:
"White" 白 (しろ, Shiro) is the intelligent (and kind) one, one of the surviving failed works and "Indigo" 藍 (あお, Indigo) who is the rough (yet also kind) one, another one of the surviving failed works. She recalls that there is another "sister," but mentions that she has since disappeared (died).
The current Female Mage that is an ally of the Hero, is considered a disgrace to the Association and has been marked for termination. Perhaps by the Association's newest artificial hero plan masterpiece, the magic-absorbing "Red (赤)" battlemage.
