Nagisa Kai is a high school student whose ordinary life shifts dramatically when approached by the robotic entity Iczel during an alien invasion led by Chaos and Cross. Their goal is to strip Earth's resources before destroying it. Initially reluctant to accept the responsibility, she hesitates to merge with Iczel and become the battle-suited warrior Iczelion. Her primary aspiration before this intervention was to become a professional wrestler.

Iczel selects her specifically to defend Earth. Merging forms the orange-armored Iczelion, granting enhanced physical capabilities rated at planetary scale or higher, matching durability, and speed exceeding light. The armor provides defensive barriers and offensive weapons like beam swords and hand beams. A specialized Burst Acceleration Mode allows temporary speed boosts. Full power access requires synchronization with Iczel, leading to early conflicts due to Nagisa's stubborn preference for wrestling-style physical attacks over Iczel's advised energy tactics.

Her development centers on overcoming fear and reluctance. Early synchronization breaks under pressure cause failures and vulnerability. Witnessing threats to others galvanizes her resolve. Key turning points include confronting Cross to protect allies, culminating in a decisive drop kick and back-drop maneuver that destroys Cross's physical form. This victory signifies acceptance of her role and embracing teamwork with three other Iczelions: Nami Shiina (Black), Kiiro Iijima (Gold), and Kawai Kawai (Silver). Their combined efforts defeat a giant Void fusion and repel Chaos's final assault.

Her journey exists within a self-contained narrative with no continuation beyond the OVAs. Radio dramas feature a different character, Nagisa Kasumi, as Iczelion, confirming Nagisa Kai's story as an alternate timeline unrelated to prior Iczer series. Her journey concludes with fully embracing her identity as Iczelion and commitment to Earth's defense.