Karin can learn sword arts by collecting Nibelung Chapters. You can start collecting it as soon as you get Nibelung Prelude at Paris. The way you get the Nibelung chapters is not character specified, but based on a fixed sequence, except Scene 9 and Finale. This means that if you talk to a character that is able to give Nibelung Scene, he will give you the Nibelung based on this sequence: Prelude> Scene 4> Scene 7> Scene 2> Scene 5> Scene 8> Scene 3> Scene 6> Scene 9> Finale.