Karen from the anime B't X is a former Spirit General of the Machine Empire, where she once guarded the western domain. Orphaned and taken in by the Machine Emperor, she was trained by the Empire's foremost warrior, Kaos, who regarded her as a granddaughter. Her early life was dedicated to the Empire's service, rising to become one of its legendary fighters. However, her loyalty was shattered when she discovered the true nature of the Empire's ultimate creation, the B't named Raphael. While witnessing a demonstration, she saw Raphael devour a group of children, with only her younger sister Karin surviving the ordeal, a trauma that caused Karin to lose all memory of Karen. Realizing that the Empire's goal was world domination through destruction, Karen became a traitor. She secured a tissue sample of Raphael, entrusted it to the Spirit General Hokuto for study, and fled the Empire, leaving her own B't, X, behind after he was critically damaged in the escape.

Barely surviving the pursuit and a confrontation with her former comrade Fou Lafine, Karen washed ashore on Kamui Island, where she was taken in by a young Teppei Takamiya. Seeing a unique inner light or shining charisma in Teppei, her own lost hope was rekindled. For the next five years, she lived in hiding on the island, dedicating herself to training Teppei as a warrior to one day stand against the Empire. During this time, she also used her engineering skills to build a new B't from spare parts, which she named Shadow X, modeled after her former partner. She crafted the Messiah Fist, a powerful gauntlet made from the tissue sample of Raphael, and gave it to Teppei to aid him in his quest after his brother was abducted by the Empire.

In terms of personality, Karen is depicted as a formidable and resilient warrior with a strong sense of justice that ultimately outweighs her sworn oaths to the Empire. Her betrayal highlights her moral conviction and her willingness to sacrifice her status and safety to prevent a greater evil. As a mentor, she is stern and focused, driven by the urgent need to prepare Teppei for the immense danger ahead, yet she is also inspired by his determination and purity of heart. Her key relationships are central to her character. She shares a deep bond of loyalty with her original B't, X, whose revival by Teppei's blood links them all together. Her relationship with Teppei evolves from rescuer and charge to a profound master-student dynamic, with Karen passing on her combat knowledge and her own hopes for the future. She also has a complex past with her fellow Spirit Generals, including her reluctant battle with Fou and her cooperation with the scientist Hokuto.

Throughout the story, Karen acts as a crucial supporting figure and mentor from the shadows. Her development sees her transform from a loyal general of a seemingly just cause, to a hunted traitor, and finally to a determined mentor who places her faith in the next generation. In the concluding events of B't X Neo, she plays an active role in the final battle against Raphael. Having discovered the Great Light energy source on Kamui Island, which she believed could counter the monster, she channeled it to empower Teppei. In the end, Karen makes the ultimate sacrifice, giving her life alongside the other Spirit Generals to ensure Raphael's final defeat. Notable abilities include her immense physical strength and skill in combat, where she wields a three-sectional staff (sansetsukon). She is also a highly intelligent and capable engineer and inventor, able to not only construct a functional B't from salvage but also to create advanced weapons like the Messiah Fist. A large X-shaped scar on her back, a remnant from a past battle treated by Dr. Zaji, serves as a permanent physical mark of her history as a warrior.