the grim reaper is most likely the fox onimaru. in the op world there is no tendency towards mythical beeings like the grim reaper to exist, so it is not a character that naturaly looks like that (we know Brook is somewhere else). Except Brooks devil fruit there is no known tool to turn a character into a skeleton like that, but there is one character in the current arc that can change his appearance to whatever he wants and that is onimaru (+Zorro is the only active charakter that has a plot-connection to onimaru). Onimaru is probably there to help zorro and changed his appearance to look like the grim reaper in order to scare off kaidos men.