Lydiane Didier is a young prodigy from a wealthy European industrial family, the Didier Heavy Industries. Despite being of elementary school age, she possesses a PhD-level intellect and works as a freelance hacker, moving in the same circles as Asagi Aiba. She serves as a supporting character in Strike the Blood II, where she is introduced as a co-worker of Asagi and the AI Mogwai in the Security Department of the Itogami Island Management Corporation. Lydiane is employed by the Taishikyoku, a powerful intelligence agency, and is tasked with managing the LYL program, a control system for a supernatural entity. Her primary motivation is to protect the systems under her charge, and she sees Asagi as both a hacker rival and a potential threat to her objectives. In Strike the Blood II, she directly confronts Asagi by challenging her to a hacking duel, demonstrating formidable technical skill and a determined, competitive personality. She later assists Asagi in logistical matters, such as helping her leave the island during a crisis, indicating a pragmatic allegiance rather than outright hostility. Lydiane continues to appear in subsequent seasons, where she remains a recurring ally and occasional adversary within the story's technological and supernatural conflicts. Her notable abilities include elite hacking expertise, a deep understanding of complex control programs like LYL, and the capacity to operate in high-pressure intelligence operations.