Lydianne Didier works as a freelance programmer for the Manmade Island Management Corporation, partnering with Asagi Aiba. She presents a youthful appearance with long hair often in twintails and a distinctive red uniform reminiscent of a school swimsuit. Her personality displays a bubbly and childlike enthusiasm, particularly when operating her specialized red tank in combat. She occasionally addresses Kojou Akatsuki with familial terms like "big brother" or "oni-chan," suggesting playful admiration. Though possessing mid-level combat skills, her primary strength lies in computing and programming expertise. Initially introduced as a computer expert who both admires and competes with Asagi Aiba, Lydianne's role expands significantly during the "Swords-Shaman of Shadow" arc. Here, she reveals her affiliation with the Taishikyoku organization and takes charge of managing the LYL control program. She directly challenges Asagi to a hacking duel to protect the terrorist organization's systems. Her allegiance shifts during "The 4th Primogenitor on the Run" storyline, where she assists Asagi's attempts to leave Itogami Island. She collaborates with Mogwai and later the convict Meiga to facilitate Asagi's escape via aircraft, marking her transition from adversary to ally. Lydianne continues her involvement in critical off-island operations, traveling to the mainland alongside Asagi in their distinctive pink tank during the Lake Kannawa crisis. Her relationship with Asagi evolves from rivalry into a cooperative partnership, evident in their joint efforts during major missions. Visual media from later seasons confirms her ongoing active presence in story events.