Specifically Aris and Kei(student). They're both children of the nameless priest(parent) and after Kei got her body they are matching in appearance. Her state when possessing Aris is closer to that of conjoined twins, or just regular twins when its drawn as 2 bodies.
As for their persona introduced at different part of story, it can be reasoned as awakening as Key has always been there with AL1S.
We've had 3 twins in the series, yeah but how about another?