The reason I disagree with HHWBehind as being Death is first It's from the Outside. It's not supposed to even be here unless summoned. So I don't see Behind as such an important part of our reality if It's not even supposed to be here.
Another reason is It's one of three while Death seems more a single Being to me.
but was it always outside? That was ritual magic they used to summon him, structured, orderly... Something that exists and has power of it's own by that existence.
As for being a singular being, even the horseman death had his fellows, though just like here, none were his equal. I'm still convinced, despite Macs 3 bottles, there's a connection between horsemen and walkers... I mean, technically I already know of a fourth walker who'd fit... MW, that's not a walking stick, that's a Walker Stick!🤣
And she'd be who "walks" before he lives up to the name of hwwbh. She'd tear it all down in a single step. Though it really has messed up the order in my head for that with Nemesis simply being a Walker, thought she was the Judge. But conquest and it's scales would still fit.