The Grim Reaper is the personification of Death (entity) itself, and tagwise are the exact same thing.
For the Billy & Mandy character, the tag grim_reaper_(character), grim_reaper_(billy_&_mandy) or grim can be used since the he's name IS Grim Reaper, which can also be implicated to either of the above.
Updated by jxh2154