In Coded Soul, you play as Jay, a soldier who travels to the world of Idea to save his girlfriend, May. The army has been conducting research into Monolith Gate, the door to the world of Idea. That door is controlled by Gale, a young boy who gives Jay a special power as he travels to the other side.
And that special power is -- Gaia fans have already guessed it -- the ability to control monsters by sucking up their souls. Once you've defeated a foe, you can make it into your ally for use during future battles. You can take up to two monsters with you on your treks into Idea, selecting from eventually over 100 beasts
Source = me actually http://www.gamed.nl/view/36308
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