As a Keyblade wielder, Jack Morrison was powerful and well-liked. After being brought to Traverse Town after his own world was destroyed by the Heartless, he had quickly settled into a similar role of leadership.
His team had been successful on many missions, all of them working together to combine their strengths. He missed his own world, but he fought hard for this one, never letting go of the hope that one day his own would be restored.
But, all lucky streaks eventually wear out.
Beset by tragedy once more, his team had been ambushed by an army of Heartless. He had stayed behind to buy them enough time to escape.
He fought fiercely, taking out scores of Heartless, but, eventually he was overrun, the creatures of darkness tearing him to shreds.
However, heroes never die, and Jack Morrison was no exception. His will was strong enough that he was reborn as a Nobody, in an unfamiliar place and an unfamiliar appearance.
With gray hair, a cold look in his eyes, and scars from a battle he couldn't remember, he was barely recognizable as his past self. Completing the transformation was a black cloak completely different than anything he had worn before.
The only memories of his past came to him in fragments of dreams, sometimes of his brief time leading Overwatch, others of his time as a Keyblade wielder. They all mixed together in a whirlwind of incoherence, people and places that were once important to him were nothing more but broken shards occasionally stabbing him in the night.
He didn't even remember his own name, the one thing that had stuck with him was his designation, Soldier: 76.
So, that's what he went by, just a generic name and number. That suited him fine, though, he was a soldier fighting his way to obtain a heart once more.