Coded Soul: Uketsugareshi Idea

An 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 soldier special power to travel to the other side to save his girlfriend, May. The special power grants the ability to control monsters by sucking up their souls. Once you've defeated a foe, you can make them 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.