others
While some of the einherjar in Valkyrie Profile have interconnected storylines, Mystina is one with few relationships outside of her storyline. Those individuals she's involved with do have an impact on her worth noting.
Arngrim
Arngrim and Mystina are the only two einherjar Lenneth cannot send up to Valhalla, and as a result likely spend a good deal of time together fighting with Lenneth. (Personally, once I get Mystina she's a permanent member of my party along with Arngrim.) They're the two einherjar around when Hrist awakens, and thus the ones that come to Lenneth's aid and strive to restore her. Their interactions don't consist of much more than working together for Lenneth's sake.
But is this the only time they've worked together? In the past in the Lost City of Dipan we see Hrist's einherjar, two of which look a great deal like Mystina and Arngrim. If it is them, then perhaps their partnership can work as well as it does because it's something they've done in a past life.
Lorenta
Although we never see Mystina interact with Lorenta, we do get to see her reaction to the news that she's been killed. It's a strange reaction indeed.
Graduate Student: Uh... Ahem. Lady Lorenta, she...she's passed away...
Mystina: Oh, I see.
(That little freak Lezard did it? I mean, I can't say I necessarily blame him...)
Mystina: Gods, what a relief! The old bag is dead! Ooh, Lezard. I could just eat you up! Thank you, thank you, thank you ♥- from Valkyrie Profile: Lenneth
Mystina is absolutely thrilled that Lorenta's been killed. She resented the headmistress for recognizing Lezard's talents above her own, even though she expelled him. And even though she has little love lost on Lezard, she's thankful he did the deed.
It may seem like a bit much, but for Mystina, it's pretty in tune with her character. Mystina hated Lorenta, and she's not the type to hold back her feelings regarding her death (especially not in the privacy of her room). It was Lorenta's focus on Lezard that held Mystina back, and with her dead she has a chance to shine more at the Flenceburg Academy, even if she'll never outshine Lezard. Of course, Mystina has to see for herself where Lorenta was killed and goes to Lezard's tower, and in so doing seals her fate.