At the age of 18, Karen enlisted in the OCU army as an intelligence officer and eventually became a skilled wanzer pilot, earning the rank of lieutenant. She was assigned to Huffman Island in a recon squad under Captain Royd Clive. While stationed at Huffman, she fell in love with the island itself along with Royd Clive, who proposed to her at the age of 22. The two planned to live on Huffman after their marriage. With the tension rising between the OCU and USN, their squad was sent to the Larcus District within USN territory. While on the mission, Karen is ambushed and outnumbered by USN troops led by Driscoll. With the others being far from her position, Karen's wanzer was destroyed and she was assumed KIA.
However, the truth of the matter was far more complex. The reconnaissance run at the Larcus District revealed Sakata Industries was developing CPUs that were integrated with human brains. Following the Larcus Incident, Karen became one of the many subjects used for the B-Device project. Shortly after the Second Huffman Conflict, Royd self destructed the Wanzer containing her CPU, allowing her to finally be at peace.