In short:
AR1: He tells you he is the ruler of his world, and seeks knowledge of the Darkness that plagues his world.
AR2: He experiments with the 'Darkness' in people's hearts, which then begin to collapse. Later, he finds the Haertless underneath the castle.
AR3: He ponders there existence, and names them [Heartless].
AR4: He comes up with an hypothesis: the Heartless take hearts, though he cannot prove
it. He then thinks they could be the Darkness in people's hearts.
AR5: He follows a Heartless which leads him to the Kayhole for Hollow Bastion. Ansem opens the door, causing the barrier between the worlds to collapse.
AR6: He hypothesizes that the door he opened leaded to the heart of his world, but he doesn't know what the Heartless want with it yet.
AR7: He studies the material from the meteor showers, and questions the existence of other worlds.
AR8: He constructs the machine that artificial creates Heartless, but he marks them with a symbol: they need to be distinct from the natural Heartless for his experiment.
AR9: King Mickey (supposedly) comes to Hollow Bastion, and talks to Ansem. Ansem discovers, that the meteors are 'gummi blocks', and the legend of the Keyblade.
AR10: He discovers that the Heartless desire the heart of the worlds. Ansme them begins a quest to find the Keyblade wielder, so he decides to cast off his body.
AR11: Summary of AR9. The Keyblade wielder locks the hearts of worlds, meaning they cannot be accessed by the Heartless. He chooses to send Kairi (a potential Princess of Heart) off to 'the ocean of other worlds', in hopes of finding the Keyblade wielder.
AR12: Ansem is now a mere heart, but he is not a Heartless. He discovers that in order to go over to 'the dark side', you must venture beyond the door of Kingdom Hearts.
There are still so many unknown worlds.
The present world.
The world of darkness.
The world of light.
And, The world in-between.
Where does the true paradise lie, I wonder?
AR13: Wonders where they body gos, after the heart has left it. It is niether Darkness nor Light, and 'in-between existence': Twilight. A mere shell, who is neither Darkness nor Light. Apparantly, since we exist here, they can't really be termed as 'existent'. So they are the 'Non-Existent Ones' (NEO)
Does that make sense??