There are many words that one could use to describe Mystina, but the foremost is "ambitious." What drives Mystina most is the quest for all the knowledge in the world. Though the top student in her time at the Flenceburg Academy, she never lost the drive to know more after her graduation. As such, she devoted her studies to the rainbow bridge, Bifrost, where it was said that Odin, lord of the gods, hung as he received his wisdom.
Though this ambition drives Mystina, there is a great deal more to her than simply that. Were her ambition softened with kindness, she might be a pleasant person. On the contrary, she is a self-centered narcissist, confident in herself and her abilities. She disdains others, giving no care or concern to those she sees as inferior. If they can't advance her knowledge or have slighted her in the past, Mystina doesn't have any concern for them.
There is in fact only one person Mystina respects as a fellow sorcerer: Lezard Valeth. It's implied that they were rivals during their days at the Flenceburg Academy, but Lezard was expelled for his endeavors into necromancy, taking all of the attention away from her. Surpassing Lezard, then, becomes the focus of her ambitions and the reason for wanting all the knowledge in the world.
Despite her ambition, there's another side to Mystina that makes her all the more interesting. She is uninterested in the trappings of flesh, instead preferring to wander about in her spirit form, which she can induce by entering a machine she created. She can then go wherever she likes without worrying about the restrictions she would encounter if she tried the same in her normal form. For Mystina, it's a form of escapism. While she wants to be recognized for her knowledge, she's not (at least not as much as she would like) and as a result, she sees no point in wasting her time in the material realm. Since she can't change things (especially not with her less-than-shining personality), she finds escaping from it all easier. In her own words:
Reality is so boring. I'd choose to sleep forever, if it'd mean I'd never stop dreaming...
A strange sentiment from such a narcissistic woman, perhaps, but it's all Mystina knows how to do. She can't surpass Lezard, so she seeks all of the knowledge in the world she can get.
Mystina may act like she cares only about herself, but she's not the best judge — she's well aware of her inability for self-analysis. As a result, what she doesn't realize is that she uses these traits to protect herself from letting anyone in. Because no one understands her, Mystina doesn't let anyone get close enough to see more into her heart. As a result, the acceptance she craves so much eludes her. She is capable of compassion and understanding, but by and large these are not traits she would show to anyone on purpose without cause.
In essence, then, Mystina is a collection of contradictions: an ambitious, narcissistic sorceress who seeks all knowledge yet finds reality boring and hides behind a mask of disdain while seeking acceptance. She is capable of changing her ways, but only if she was to realize her faults and was willing to change them to achieve what she wants most. As it stands, it's not likely to happen any time soon.