A Card Quester named "Ichibanboshi no Rei" is searching for the Ultimate Battle Spirits, a treasure somewhere in the galaxy. With him are the mini dragon Mugen, a cook robot named Salt, and the April siblings Laila and Rikuto. Rei transforms into Zero while on the battlefield, using different colors and Ultimate cards to defeat his opponents. While he heads toward the Ultimate Battle Spirits, he settles conflict around the galaxy, befriends various outlaws, and clashes with the Guild, an organization controlling space.

Source: Battle Spirits Wiki

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Prequel: Battle Spirits: Sword Eyes Gekitouden
Sequel: Battle Spirits: Burning Soul