Hmmmm,quite unsure about this..........ok i'll say something strange,imo the most underrated warrior inside the story is definitely Claire , if instead this nomination is about "underrated by the readers", i don't really know who nominate for now,i'll think about it some more.....
Flora is indeed a good suggestion, I would have loved to learn more about her back-story. In that category, the other captain Veronica is even more mysterious, though she makes less of an impression. Of the lesser Lost Seven members, Tapitha and especially Yuma/Uma is indeed interesting for their development in the shadows of Clare, Miria and the Helen-Denève duo. However, their stories do get lots of coverage, unless that of the third: Cynthia. WWe don't get much of a back-story (fleshing out her relationship with Veronica for example), and we mostly see her healing and detecting abilities and not much so far of her fighting abilities, which certainly improved the same way Yuma's or Tabitha's did. So I nominate her.