Gravion

Eiji Shigure receives a letter from his lost older sister Ayaka Shigure in Gravion. In response to its message, Eiji follows his hunt for her to the enigmatic billionaire Klein Sandman, and while he is throwing a party for Earth Federation Alliance (EFA) leaders, sneaks into his gigantic Saint-Germain Castle to discover her. During a mission of Sandman's Earthgertz crew to repel the invading mechanical extraterrestrials known as the Zeravire, he ends himself driving the G-Attacker, one of a series of five, secretly created by Sandman himself, vehicles nicknamed Gran Divas. Eiji is informed by Sandman that his sister, who also possesses the G-Factor, was secretly a member of Earthgertz and that his special G-Factor enables him to operate devices like the G-Attacker. Sandman was obliged to deliver the letter to Eiji in order to bring him there in order to bring him onto the team after she left on a mission. When the Gran Divas and the Gran Kaiser mecha are combined to create the titular super robot, Gravion, Eiji joins Earthgertz, the planet's only defense unit capable of stopping the Zeravire from killing humanity, despite only having an interest in recovering his sister.