Well, if you want to get into spoilers, we do know a bit more. The Reaper seems to be a grown-up Kokujo--his full name is outright stated later on. I'd say we still don't know if he's 100% one of our Kokujos from either Shobu manga or anime continuities, or if there's some alternate universe shenanigans going on, but the reaper's name is definitely Kokujo Kyoshiro