I honestly don't really know what the working story is, I could talk about it to Pyon though it's not really the biggest deal.
My kinda idea was that Meredith sorta just runs a unit that is basically, all the misfits of the Misguided Children or people that join up but the more "normal" units wouldn't take them in a million years for whatever reason. Meredith had this kind of experience first arriving there I feel, seeing as she was from Dragao and fought for Dragao and earlier on she'd have been easier to pin as "The Undying" since it was more recent when she joined up, so no one was really all that willing to accept her at the time, so she just ended up running her own unit of all the folks like her within the ranks of the Children lol.
Of course, this is just my head canon and not really something that's set in stone. I just figured she was sorta the unit leader ((and people have already said she basically is lol.)) I just figure it explains it in an interesting way as to how all these misfits ended up in the same place. ((and I feel it gives and interesting characterization point to Meredith in that she gives other people second chances, or is quicker to give them the benefit of the doubt then others due to what's gone on in her life.))
Just my thoughts on it lol. Nothing is really set in stone though. I'm not really even sure how the Children are structured and if that even makes sense in the context of them XD.