Are you talking about a show or just a girl? I thought you were talking about a show but everyone else has been posting characters so I'm confused. But I'll go with Rin from Mnemosyne and I'll also go with that show to answer either question.
for girl butt-kicking anime, I'd go with Black Lagoon or Kara no Kyoukai. For magic girl anime, I'd go with Puella Magi Madoka Magika or Tokyo Mew Mew. But I think my all around favorite is Black Rock Shooter :)