Lezard
The relationship between Mystina and Lezard Valeth is one of the few relationships in Valkyrie Profile that we get to see a good bit of. On the surface, it's fairly simple. They were both students at the Flenceburg Academy, and though Mystina graduated at the top of her class, Lezard was the one who got all of the attention by being expelled.
Character summary
Mystina graduated from the Flenceburg Sorcery Collegium at the top of her class, and continues to work there as a researcher. [...] She hates Headmistress Lorenta, and burns with antagonism for Lezard, her expelled classmate.- from the Valkyrie Profile Material Collection
Despite this antagonism, there's a certain camaraderie between them — Lezard calls her "Mysty," and she doesn't seem to mind the nickname. During their discussion of elves during Mystina's story, Lezard seems rather pedantic in his explanations, but Mystina easily keeps up and prods him to move along. In fact, it's because Mystina is on the same level as he is mentally that makes her existence enough of a threat that he disposes of her.
And yet, when their goal is the same — to save Lenneth — they can put their differences aside and work together.
As much as Lezard influences Mystina, it doesn't work both ways. He saw her as nothing more than a pawn to be used, an outlet to explain his evil plans to, and a potential problem to eliminate just in case. I don't think he cared even half as much about her as she did about him. When it gets down to it, her hatred for him is more about how he represents an obstacle she simply can't overcome, no matter how much she tries.
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
Surpassing Lezard was a dream she couldn't make real, so Mystina turned away from reality entirely. And in the end, that's what caused her death.