What if you see the grim reaper?

Do you mean in The Sims games or reality? I'll give you 2 explanations:

1. Reality - The grim reaper isn't real. It's just a gimmick on TV shows and Video Games.

2. The Sims Games - One Sim will softly cry for the dying Sim. And nothing will happen except if there is only one Sim left. Mr. Death will explore the house, go to the toilet etc.