>>723601853 (OP)
Even the lore enthusiasts don't know shit about him. He's completely enigmatic and we likely won't know any more until at least KH4, if we get anything about him there. All we know is he's got this 57D Backgammon strategy spanning countless generations with Luxu (currently Braig), who's been looking after MoM's keyblade (who up until recently has been acquired by Xehanort) since the start, being his main centerpiece. He's as autistically fixated on destroying any and all darkness, but how he plans to go about the whole thing is still a mystery. Playing through UX only helps so much, and his current wherabouts or state of his plan are completely unknown (though he's seen in the teaser at the end of KH3 in Quadratum alongside Sora and Riku).
I like to believe Demyx is his Nobody.