Master of Masters: The darkness has thirteen main parts, and among them are seven particularly deadly ones.
I don't even have a problem with Xigbar being strongly connected to Luxu. Like, here's an idea and maybe this
is what Nomura originally had in mind. We know that Darkness originally gave up its physical form and is looking for new ones. The MoM bestowed on his apprentices new names (the names of the deadly sins) and sealed the seven deadliest Darknesses inside them and himself.
It would make perfect sense if the same Darkness that was hosted inside of Luxu was now inside of Xigbar. Braig
was a fool desirous of the Keyblade's power, so he was capable of hosting the Darkness of Lust, Luxu. And maybe Saix hosted Wrath in him, Xehanort hosted Pride. The other Norts hosted the other deadly sins. I could have easily accepted Xigbar being possessed by the Darkness Luxu. But by the
human Luxu? That's a stretch.