I mean already know about Kayto suffering from PTSD and everything but it has made me think. A lot of characters in Sunrider do act really strong when it comes to their personality type to the point where it could be considered a mental disorder of some type. And we never get to really see the story from the other characters POV's.
It's hard to judge what could qualify as disorder and what is simply exagerated behaviour due to the genre of this game. But I agree: most (if not all) of the characters have some reason or maybe even a single event that formed huge parts of their personality.
If we take Sola as example: even though her latent insecurity and permanent death-wish are by no way natural, I don't think it qualifies as disorder. Even though we already know more about Sola's backstory than about any other character, I hope to see even more about her in the beginning of the next game.
I think if any character actually qualifies to have a disorder, I'd say it's Ikari. She lost her parents when she was a child and did gruesome things for a living. Let's see whether we get more of the cynical Ikari of MoA in the next game instead of the more happy one we had in LD (even though that was fun and interesting, I think her being sarcastic makes for a more interesting character. Also, she has more than enough reason after losing the safe home she had on the Sunrider).