escapism
Mystina dies a metaphorical death long before her physical one: by taking to her spirit form. In Valkyrie Profile, most spirits parted with their flesh are considered dead, but Mystina induces this condition intentionally. To be able to do such a thing means she must be highly intelligent — after all, she is the only other human Lezard considered a threat. My guess is that Mystina created the device she uses to get into her spirit form.
The real question, though, is why. Why would she want to be a living ghost? A typical motif of ghosts in fiction is that they could not move on from what happened in their lives, unable to shed their mortal coil. Yet this is precisely the condition Mystina chooses to experience. Does it mean, then, that she has no attachment to the world?
Though Mystina's story in the game raises these questions, it does not answer them. It's in supplementary material that we learn why it is Mystina shuns reality so.
What happened to prompt her attitude that dreams are better than reality?
It would seem to be because a genius called Lezard Valeth exists in the real world, and the attitude comes from a feeling that there's no way she can be the best while he exists. Mystina is, one can say, like a female version of Lezard — an egoist, a narcissist, and good-looking (laugh). Mystina harbors quite a grudge against Lezard.- from the Valkyrie Profile Material Collection
Even though Lezard was expelled from the Flenceburg Academy, he was by far the star pupil, and stole the spotlight. For Mystina, Lezard's existence means that once she reaches the pinnacle of her potential and can go no further, Lezard will still be superior. It's demonstrated in the game by the fact that Lezard found the Philosopher's Stone, the "ten-billion-page codex," while Mystina is still just a scholar in Flenceburg.
I think Mystina's spirit-walking stems a lot from this. She's not recognized for her endeavors in reality, so what's the point of being there? Spirit-walking is an escape for her, one she finds easier than trying to change things. But it's not just through spirit-walking Mystina hides herself; she does it just as much with others.
The woman who wanted to be chosen
Behind her stubborn attitude, Mystina has a desire to be understood. She was born with an extraordinary intellect and beauty, so one may expect that she'd not lead a normal lifestyle, but she didn't choose to walk the solitary road of academia. One can't help feeling that her egoism is just an act. Her narcissism might be something she clings to as a manner of self-defense. In truth, with the exception of herself, one wonders if there's anybody who understands her. She hasn't received all of the acclaim for reaching the height of scholarship at the Collegium, however. The Headmistress recognized another student (Lezard); so Mystina saw it.- from the Valkyrie Profile Material Collection
Mystina's more likely to put up a front than anything else. A lot of her personality is how she acts instead of how she really feels. Take for example the very first lines we hear from her.
Graduate Student: Oh, ah... Miss Mystina. Whatever are you doing, wandering around at this hour?
Mystina: Oh, this and that. Basically, I'm consumed with melancholy after a particularly debauched midnight tryst.- from Valkyrie Profile: Lenneth
Instead of giving a straightforward answer to such a simple question, Mystina deflects it. She lets no one into her heart, because they simply don't matter to her. There's one exception to this: Lenneth. The Material Collection says it best:
Her devotion to Lenneth
Blessed and surrounded by friends and lovers, Mystina doesn't actually think reality is boring. She knew her heresy was unacceptable in Flenceburg. More than that, she realized her own solitary existence would be painful. Day after day she endured appearing as if odd in contrast with her surroundings. Even though she craved affection, her attitude toward others was sharp because she didn't care about them. However, upon analyzing Mystina's personality, Lenneth accepted her for who she was. Perhaps because of this, Lenneth is the only one she puts her faith in. Even during the disaster involving the valkyrie's body, she made it clear that her loyalty was for Lenneth, not Hrist — because she likes Lenneth.- from the Valkyrie Profile Material Collection
For all that Mystina tried to be recognized, only Lenneth really gets her and more importantly accepts her. As far as Mystina is concerned, that's something worthy of her respect and faith. Mystina's not the type of person to pin all of her hopes and dreams on Lenneth — she says herself she doesn't need stupid promises — but it is enough to make her break out of her self-imposed shell of egotism and trust her. Escapism isn't really what Mystina wanted, in the end — just recognition.