As of the most recently translated chapter, c761, they have not. Any proper reply to this question would be a spoiler. If anyone has the answer, please remember to use spoiler tags.
Huh. Well, they do turn up from time to time. The novel keeps track of their life in Klein's absence. But, yeah. No reveal so far.
First of all spoiler...And in fact i think they Spoiler: Spoiler will find out when hes Sequence 1,0 or even (Which is possible)they go in the negative sequence which i think exists lel