He’s Kirby Jenner. He claims to be the fraternal twin of Kendall Jenner, though I think it’s safe to say he isn’t.
His mission in life appears to be inserting himself into the photos of various Kardashian/Jenners (though primarily he’s seen with sis Kendall) and then graciously post them on his IG where the rest of us can take them in.
His perfect poses and deadpan expressions make his randomly staged photos seem down right plausible. I tried to just choose my favorites to share: