Gravion

Gravion (超重神グラヴィオン, Chōjūshin Guravion, lit. "Super Heavyweight God Gravion") is an anime with 13 episodes in 2002 and another 12 in 2004, totaling 25.

Plot
In Gravion, Eiji Shigure is sent a letter from his missing older sister Ayaka Shigure. His search for her leads him to Klein Sandman, a mysterious billionaire and Eiji secretly infiltrates his enormous Saint-Germain Castle while he is hosting a party for Earth Federation Alliance (EFA) leaders, in order to find her. Unfortunately, he is tricked into piloting the G-Attacker, one of four Gran Divas, in a mission to defeat the mechanical extraterrestrials called the Zeravire. The Gran Divas are machines secretly created by Sandman himself, who states Eiji's unique G-Factor allows him to pilot it. Although reluctant, Eiji becomes part of Sandman's Earthgertz, the planet's only defensive team to prevent the Zeravire from destroying humanity, by combining the Gran Divas with the Gran Kaiser to form the titular mecha, Gravion.

In Gravion Zwei, the Zeravire invasion seems to have stopped. However, when it rises back up all of a sudden, the Earthgertz and its trump card, the super robot Gravion, is needed to defend the Earth again. In the meantime, the EFA creates their own weapons to counter the Zeravire, the Gran Trooper. As they attempt to fend off the Zeravire, the Earthgertz and the EFA must set aside their differences, confront their pasts and unveil the mysteries of the Zeravire, in order to end their impending threat once and for all.