Unfortunately, there's little backstory. Not all the aliens are monsters or out for conquest. E.g., the White World is ruled by angels. Some came to Earth as refugees or even actively oppose their homeworld's agenda. But none can survive for long away from the energy provided by their respective portals. Technology was developed to capture and bind aliens to portable energy devices, forcing them to fight for the holder's side. A mysterious shrine priestess bestowed a few "cards" to hand-picked champions, who with their allies try to save Earth from all-out war and devastation. A nice action-adventure series.
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