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Kingdom hearts birth by sleep - A very fun game with rather good storyline.
It's about these 3 people, Ventus, Aqua and Terra, being very close friends to the point of treating one another as siblings, training to become something known as the "keyblade master". However, in the world of this game, darkness and light are very significant things, so a person who fell into darkness can become really evil. So, keyblade masters must be pure and filled with light in their hearts. So, Terra, which had darkness in his heart, could not become a keyblade master. Though, he was sent to other worlds (this story is made up of lots of worlds) to carry out a duty to find a missing person. Somehow, his friends heard rumours about him doing evil stuff.

This game is filled with the discussion of light and darkness. And the worlds that i've mentioned earlier are actually disney worlds and fairytale worlds. You get to control a character and see how they affect these worlds and stuff. It's fun. It's a psp game anyway.