A seventeen year old who becomes entangled in a battle for Earth's survival, Eiji Shigure is the central protagonist searching for his missing older sister, Ayaka Shigure. His quest begins after receiving a letter from her, leading him to the estate of the enigmatic billionaire Klein Sandman. Believing his sister was last seen in Sandman's service, Eiji infiltrates Saint-Germain Castle, only to be caught up in an attack by the mechanical alien force known as the Zeravire. During the chaos, he is recruited into the Earthgertz defense organization as the pilot of the Gran Diva known as the G-Attacker, a role he reluctantly accepts with the primary condition that he will leave as soon as he finds his sister. By the events of Gravion Zwei, he has become the pilot of the Geo Mirage.

Eiji is characterized by a straightforward, hot-blooded nature and a notably short temper, which can sometimes make him easy to manipulate. He is easily annoyed by those he perceives as carefree or clueless, particularly his teammate Toga Tenkuji, who lived a sheltered life before Eiji arrived. He also clashes with fellow pilot Luna Gusuku, who often teases him. Despite his impulsive tendencies, he is fundamentally reasonable and his rash decisions are often grounded in logic. His deep care for his older sister is a defining trait, serving as his primary drive. Though initially seeing his role as a means to an end, he develops a close friendship with his teammates and becomes fully committed to stopping the Zeravire threat, recognizing its reality and danger.

Within the story of Gravion Zwei, Eiji serves as a key member of the Gran Knights, the elite pilot team operating the super robot Gravion. He is the pilot of the Geo Mirage, a component of the more powerful Sol Gravion. Although he is the protagonist, his role is that of a supporting leader and foil to Toga, who is the primary pilot of the Gran Kaiser. Eiji’s contrasting personality helps Toga grow as a person and learn restraint in combat, serving as a voice of reason. His primary motivation remains finding his sister Ayaka, a mystery that is central to the plot of both seasons. Key relationships include his rivalry and friendship with Toga, his bickering dynamic with Luna which masks a deep mutual respect, and his complicated connection to Mizuki Tachibana, an older teammate who was a friend of his sister.

Eiji undergoes significant development over the course of the series. He evolves from a reluctant participant focused solely on a personal goal to a dedicated hero willing to risk everything for the safety of Earth and his team. He grows into a pilot capable of piloting the advanced Geo Mirage, a dangerous machine entrusted to him because he has earned the trust of both Sandman and Toga. His journey directly and indirectly influences the members of the Gran Knights, helping them to confront their own pasts and unite as a more effective team. His abilities are notable primarily for his unique G-Factor, a natural aptitude that allows him to pilot the advanced Gran Diva machines. While he is a skilled and courageous combatant, his primary function within the Gravion is often as a support unit, providing energy and firepower for the super robot's most powerful attacks.