Nagisa Kai is a high school student aspiring to become a professional wrestler who is unexpectedly chosen by the robotic entity Iczel to defend Earth from the alien invaders known as Geas, led by the android siblings Chaos and Cross. Initially resistant to this role, she prioritizes maintaining a normal life and expresses significant reluctance toward merging with Iczel to become the armored warrior "Iczelion," reacting with fear, avoidance, and attempts to flee battles.

When forced into combat, she uniquely integrates her pro-wrestling techniques—executing dropkicks, neck jabs, and backdrops—into her fighting style, often bypassing Iczel's conventional energy-based attacks. This unconventional approach causes friction with her robotic partner, who questions its efficacy. Her resolve solidifies when bystanders or fellow Iczelions are threatened; witnessing these dangers galvanizes her protective instincts, ultimately overcoming her hesitation. A pivotal moment sees her decisively defeat the villain Cross using a high-altitude backdrop, demonstrating her full commitment.

Synchronizing with Iczel grants her Iczelion abilities: generating beam swords and hand beams, deploying defensive barriers, and accessing a temporary "Burst Acceleration Mode" for enhanced speed that leaves trails of light during movement. She exhibits combat durability comparable to planetary-scale threats, based on comparisons to earlier Iczer-series warriors. Her personality evolves from fearful indecision to determined leadership, culminating in her embracing the Iczelion identity and urging allies to address her as such during battles. Radio dramas associated with the franchise focus on a different character, Nagisa Kasumi, and do not feature Kai.