The Grim Reaper is a common element in culture and history. As a personified force it has been imagined in many different ways. In some mythologies, the Grim Reaper causes the victim's death by coming to collect them.
The Grim Reaper is a skeleton draped in a torn robe. His rib cage and head are visible, but nothing seems to be behind them other than darkness. His eyes are bright red, and are easily spotted in the darkness. He wields a large scythe (so large that it doesn't fit in the screenshot). And levitates around the place. His taunt sounds like more like ambience, and less like a growl or dialogue.
His ability cannot be used until he has killed a survivor and that survivor bleeds out, then their soul is retrieved. Using his ability, he can damage all nearby survivors with a large dark cloud of smoke. This damages over time, but can be used to slow down weaker survivors. As a survivor affected by this, you see no health bar with the damage dealt, because your max health is lowered, but your health bar is still full.
The number of collected souls can be seen by light dots on his scythe's blade.