Is Zero...special?

[Alleluia]

But friendship is not a foreign concept to voidsent. Just Zero.

Zero is weird among voidsent. The only one who seems to have trouble interacting with others.

True, she is. I don't think it has to do with any medical condition, though. Unless you count partial amnesia or purposeful self-deception as a medical condition.

We know Y'shtola considers Zero's response about how she avoided becoming like other voidsent to be sus from her "... Is that so?" reaction to it. I am guessing one of the above is the reason for that. And if Zero isn't even remembering everything about herself or her past (due to suppressing it or w/e) then not remembering what a friend is isn't too far fetched. B/c as others have said, actual friendship isn't really present much in the 13th, it seems.

As for the Gruffs, and arguably the other members of the Halloween troupe, they have been on the Source and interacting with Source culture and people for quite a while. And they were all normal people once, themselves. Being in the Source's environment, free of the pressures the 13th has, has probably allowed them to develop socially enough to create things that look more like a family unit. And even as it is, one could argue the Halloween troupe is a kind of mobile domain with the orange pumpkin headed guy as its leader.