Ken has issues that could have made him so much worse.
Just a guess but Mark might have some kind of emotion problem.
"Problems" might be the common thread but that's so vague I'm not sure about counting it myself.
Ken seems to have a bipolar disorder or double personality. For now i would say he is more of a loose cannon. It's hard to say how much of what he does is him.
Mark though, if he is indeed the perpretator of things the story has implied there's no excuse. Emotional trauma doesn't absolve you from culpability, it merely gives context.
We often equate character routes to romantic routes, but that doesn't have to always be the case. It's not impossible for Grey to not end up with any of the main characters, because that might not be the point to the story. Considering what's been presented so far, the goal seems for Grey to save those characters, potentially from themselves, and in turn learn to value his own.