Out of the current 15 episodes in the webtoon, the Grim Reaper only appears in Episode 3 in order to escort Aru to the Underworld. Even then, something tells me that he may come back in the future so be ready. 

In that one episode, the Grim Reaper's personality is very clear. He doesn't like to talk excessively and gets annoyed at the number of questions Aru asks upon meeting him. You could say that he's a cold and tired guy. He has very pale skin and dark circles under his eyes. Despite that, Aru stills fawns over how hot he is when she first meets him. Even though the Grim Reaper seems like he's just done with life, when he's sending the deceased to heaven, he actually wears a genuine smile on his face.

Medium to Medium-Low Tone I imagine a tired voice, maybe a little hoarse, but still a voice that someone would want to listen to and not want to cover their ears in pain lol.