Carol was born into House Remendis, a noble warrior family known for producing exceptional swordsmen who'd served House Astrea for generations. At 14, she was assigned as a personal attendant to Theresia right around the time—two years before the Demi-human War escalated—when Theresia awakened her "Divine Protection of Sword Saint" at just 13 years old. Their relationship started rocky, but after some unspecified incident, they became genuinely close friends. Theresia apparently trusted Carol with things she couldn't share with anyone else.
After Theresia's early battle failure, Carol stepped in as her stand-in for the next three years, handling duties like guarding Roswaal J Mathers, with whom she developed a real rapport. This was also when she linked up with Wilhelm, Grimm, and the rest of Zergrev Squadron. She and Wilhelm took a while to warm up to each other, but she and Grimm hit it off immediately—and feelings developed between them over time. Though Carol never formally joined the military and mainly supported others throughout the war rather than taking direct command, she saw action in most of the conflict's major engagements, including the Battle of Aihiya Swamp and the Battle of Luginica Castle. At Luginica, she made her mark by severing Sphinx's arm—a decisive blow against one of the Demi-human Alliance's top leaders that helped secure a crucial victory.
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