Lydianne Didier operates as a freelance programmer contracted by the Manmade Island Management Corporation, collaborating with Asagi Aiba to oversee the island's critical systems. She presents a youthful visage accentuated by long twintail-styled hair and a unique red uniform reminiscent of a school swimsuit.

Her demeanor radiates energetic, childlike enthusiasm, particularly when piloting her robotic tank into combat situations. She shares a familiar rapport with Kojou Akatsuki, addressing him affectionately as "big brother" or "oni-chan." Professionally, she wields advanced programming expertise that positions her as both an admirer and rival to Asagi’s technical prowess.

In combat operations, Lydianne deploys a specialized red tank, though her tactical proficiency ranks as intermediate rather than exceptional.