The vast majority of Negima fics are either Negi/Asuna, Negi/Nodoka, Negi/Eva or Konoka/Setsuna, so I guess you could say that EVERY OTHER GIRL is underused, pretty much.
Heck, I haven't seen a single Negi/Kazumi or Negi/Chisame fanfic either. And Chisame is actually one of the mos underused girls even in the manga, let alone in fiction - she almost vanished besides comic relief after the whole Chi-u fiasco.