So...
Is the Skill [???] just something that makes him act like the other idiots around him?
Because Dia is a fucking idiot; "I HAAAVE TOOOOO use a sword!!!"
She doesn't have to use a sword, at all; even if her family is swordsmen, and it's required for her to gain some bullshit title, it doesn't fucking matter if she can kill them with magic; and, it gives her the power and money she says is a requirement for living and breathing (which it isn't, even if the author tries to pretend otherwise).
So, she has this absolutely fucking psychotic need to use a sword, and he has a psychotic episode if people talk to him, touch him, or exist near him.
He wouldn't be able to walk around outside in Japan, wouldn't be able to care for his sister, wouldn't cook, wouldn't enter a hospital; he cannot function outside of an assisted living environment if he normally has a psychotic break every time somebody asks him how his day went.
This means that he wasn't like this before being transferred.
Ergo, Skill [???] is likely the cause, and it is probably something that makes him act like the fucking weirdos who live in this world; because these people are NOT NORMAL.
In fact, it's kinda creepy; like a dream where everything is the same, but also somehow completely wrong...
And that would drive you psychotic.
Hmm.