As part of the German Army Karin is ordered to capture the small village of Domremy, where there are rumours of the existence of a demon. Upon entering the village, her squad is quickly defeated by the creature. As Karin is slowly falling unconscious on the floor, she catches a glimpse of the demon walking away and shape-shifting to a human.
After this event, she is ordered to accompany the Cardinal Nicolai Conrad, to retrieve a holy artifact. Upon retrieving it, they return to the village to have another go against the demon. During the fight that ensues, Karin accidentally falls near a grenade thrown by her own squad, but is saved by the very demon which they were attacking. This moment of hesitation allows Nicolai to use the artifact, a mistletoe, against the demon. After fighting some more, the enemy soldiers leave the village, but Karin stays behind, unsure of what to think.
She later finds out that the demon was in fact
, a Harmonizer protecting the village from enemy soldiers. Due to the curse caused by the mistletoe, Yuri can no longer protect the village, and it quickly falls to the fires of war. He then decides to leave, trying to find a cure for his curse, and Karin, still unsure of what to think or of what she feels, she decides to follow and help Yuri on his journey.
Fiercely independent and well-mannered, she harbors a crush (and later, a deep love) for Yuri, and sticks by his side in any situation.
Karin uses swords, particularly rapiers and sabers. She obtains Sword Art power-ups through the score of a Wagner opera, the
Ring of the Nibelungen
. Karin doesn't really have a weakness nor strength. Karin's trademark element is fire.