Huh... so what's Aria's connection with the Kenkakushuu that massacred Blue Sword's buddies with her invisible sword? There's no way they're unrelated. Also Aria probably wants to duel Alta because she heard he beat someone from the Kenkakushuu... the organization that perhaps her "sister" is part of.
It's an unfortunate necessity for the story to have opponents that can, at least theoretically, be the MC's opponent. At the same time, it makes little sense when considering that whole world. If the Kenkakashuu is as large a group as the thick pile of Wanted posters suggested, and they kill anyone as long as the customer has coin, what prevents any noble from hiring them to kill the entire royal family of any kingdom? If knights are so powerless against them, nothing could stop them. The power balance simply isn't there.