Sagan Nagisa (目渚)
A young woman whose job and past are unknown. She seems to be a scientist of some type, and is credited with creating the Zeros, who she treats like her own children (if in a "tough love" sort of way). Nagisa has a fiery rivalry with her colleague, Ritsu, who she secretly loves. This was hinted at for a long time before Ritsu's injury in volume 7.
Although the details are sparse, Nagisa has a deceased sister who resembles Youji. That may have been intentional, but the details surrounding that (and how she 'creates' Zeros) are a mystery.