Ok we know in black butler grim reapers collect souls, and my head cannon for the day is that grim reapers hoard stuff, like grell and his wired doll thing, undertaker and bodys, coffins, that sort of think, and I'm going to drag my oc into this( sorry) but she collects potions, and whitchy stuff, I think that grim reapers have to collect something, if not souls them a item, a thing, something they can hoard,
* points dramatically * EXBIT A

Grell, grell collects those creepy doll things, and he has QUITE a few from what I have seen, so grell has that collection

* points, * da taker

Undertaker, undertaker collects, coffins, body's, jokes, bones no doubt, and the list keeps growing, undertaker has been retired for a bit, so he has had time to make his collection look presentable, his shop is his collection pretty much

My oc( I know )
she collects potions and black magic stuff, candels, books, herbs, she literally has a shop full of this kind of stuff because that's her hoard, ( I have a oc book on her witch will explain this collection of hers )

So basically grim reapers are dragons, they like collecting stuff