The 8 shown plus the 3 others mentioned in Chapter 14 match the total of 11 that Kurumi describes in Chapter 8. The Empress/Brigadier seems to be personally acquainted with Asuka as a magical girl, and presumably is one of the original 11 magical girls. We can eliminate 4 of the Legendary 5—from what we've seen of Asuka, Kurumi, Mia, and Tamara, they're clearly unaware of The Brigadier's plans and/or are shown to be present in a place where she is also present. The fifth, Pei-pei, doesn't match her appearance, but we don't know anything about her, and she could have changed, so that's one possibility.
Of the 3 others we see in this chapter, physical appearance rules out two of them—Francine's frame is too small (plus she got ripped in half) and Claudia is too dark—but Magdalena is a possibility. Another possibility is that she is one of the other 3 mentioned in this chapter but not shown.
Lastly, some of these assumptions could be incorrect. She could be some other girl who knows Asuka but who wasn't one of the original 11 magical girls—Abigail and Whiplash show that new magical girls can be created. Also, it's possible that she's using some kind of appearance-changing magic, and so is one of the magical girls I've eliminated for looking too different. On that same note, because magic is involved she could even be Kurumi, or Asuka herself, even though it makes no logical sense.
So basically, I think the Brigadier is one of the 3 original magical girls mentioned in this chapter but not shown, or perhaps Magdalena. Additionally, she could be Pei-pei or a magical girl created in the same manner as Abigail and Whiplash.