Much like with Darkness Incarnate, Light Incarnate was born when a massacure of Denizens of Light happened, when Blaid killed off a large number of Keyblade Wielders after he was betrayed by a pair of Masters.
Due to the "Other Universe" being a prison for beings of the Light, it became rather easy for the Light based Unversed to form and grow vastly powerful however it found a problem, Darkness could take a physical form while Light expecially couldn't.
This forced the beast to wait over thirty years until he gained enough power and a worthy being stepped through the prison's Mirror into his artifical Universe, where he was easily mistaken as the Heart of That Universe as he was the only Light there.
Gaining a Body
Light Incarnate acting as a "Kingdom Hearts" gave Tsuki a ultimatum when his mind wandered into the "Other Universe":
- He can remain in that Universe, and be spared the great sufferings he would surely come across if he left, to go back into his Home Universe.
- The "God" would allow Tsuki to gain a "Higher Existance" at the cost of his humanity and right to live his life as he choses.
Basically, Tsuki agreed on the second decision and Light Incarnate merged with Tsuki's Heart, going dormant and working his powers throughout Tsuki's body. After Tsuki kills his father and comes face-to-face with Blaid the true personality that is Light Incarnate takes over, Blaid then commented how the "Incarnates" are in fact the reason why both Light and Dark Keyblade Wielders are blamed for wanting Kingdom Hearts, this is a later reinforced when Light Incarnate confirms that he wants to "Become" Kingdom Hearts and the only way to do that is to absorb the original. Blaid counters this by saying that when he unlocked the pathway into the Realm of Nothingness before, he had sealed the path from the Endless Abyss into Kingdom Hearts, much to Light Incarnate's fury.
Outside of Battle:
Conversation between Light Incarnate and Blaid
Before First Battle
Light Incarnate: "The Denizens of Light have become pathetic" (To Blaid) "You saw the fools. A mindless mob, drunk on fear, on a few Denizens of Darkness!"
Blaid: "To think, its a Denizen of Destruction: a natural Incarnate of Light, whom is filling their glasses!!"
Light Incarnate: (laughs) "An Incarnate? Me? Oh child, perish the thought, I am more than that! I am...Kingdom Hearts!!"
Blaid: "Many Denizens of the Light claimed that or that they were serving Kingdom Hearts' will, I'll tell you what I told them..."
Light Incarnate: "Oh? And what is that?"
Blaid: "In your dreams" (summons Perfected Z-Blade and Attacks) "just because you dream of it, doesn't make it any more real!"
After First Battle/Before Second Battle
Light Incarnate: (backs away a safe distance from Blaid) "You said Incarnates, there are others, like me?"
Blaid: "None of Light, my ancestors created Incarnations of Darkness, machines which were used to infect the civillian populace with Darkness"
Light Incarnate: "So, they are welcome members of your kind?"
Blaid: "Not at all"
David: (appears on Keyblade Rider) "I'm suprised you'd say that"
Blaid: "The Purge are the "Incarnates" fault" (glares at Light Incarnate) "You and your brother caused them to go rogue with your presence!"
David: "Kingdom Hearts told us to save this Universe!" (summons X-Blade and points it at Light Incarnate)
Blaid: "The Purge is happening and people are dying, if what you said is true, David..." (points Perfected Z-Blade at Darkness Incarnate) "Then this thing has to die!!"