He's Riku's Nobody from another world/alternate universe, which I find kinda impossible unless you take some real liberties in describing what happened to Riku during KH1, perhaps KH: CoM.
He looks similar, but he and another (the Nameless Stell- I mean Star) already pointed out he's not like that originally, meaning his form was somewhat dictated by the circumstances of he and Sora meeting the way they did.
Or...
He's Noctis's Nobody, which is only something that would be the case in a meta way, and is more likely to be true in a different sense in-universe.
Considering how this whole shebang is very very connected to how Versus XIII moved on after the troubles it had, in a meta sense a bunch of words said in-game sell this, like how apparently his heart was replaced by another's, and if he got it back he would remember somebody important and go to rescue them.
In an in-universe way, I can't see this too clearly right now, I just don't think we have enough to go on to make any real claims without serious dubious reasoning, like Shibuya taking "something" from him to give him his chance at life, potentially being his heart or memory of that special somebody.
(who, real talk, I'm starting to believe is named Sora or something, maybe I read the scenes wrong, but it doesn't at all feel like he's talking about somebody he doesn't already know, and he definitly doesn't seem to be talking about OUR Sora, so maybe the Nameless Star's original name was Sora in his world. I mean, it makes sense, considering his name is Yozora, night sky, and Sora, far as I know, is a Unisex name. Not like Versus XIII was light on the naming symbolism.)
So, maybe he lost his heart, and through his trials he developed his memory of why he was fighting? Maybe he got it back and lost the person he cared for? TWEWY definitely pulled stuff like that on the protagonist during it's playtime, so it's not a stretch to assume that was the case. Only problem, this is all assumptions. No real evidence talking about his "trials" in Shibuya would lead to understanding his whole deal.
Of course this could all be nonsensical speculation and have nothing to do with anything, but honestly there is way too much to unpack with this guy and literally everything that happens at the end of that DLC, so it's as good a theory to talk about considering he isn't the only character in Kingdom Hearts that has a Nobody earring.
"Amazing, isn't it?"