After losing a bet to Billy and Mandy over some sick hamster, the Grim Reaper has agreed to be their best friend forever and ever.
Billy is dim-witted and gross while Mandy is rather intelligent and sour-faced and has probably never smiled a day in her life.
Grim, while being king of the underworld, definitely does not rule on Earth.
The show can be gory, gruesome and gross. It originally started out being called Grim & Evil and the show had 2 separate cartoons running on one show.
The one was of course Grim and the other was the Evil Hector Con Carne.
The latter was eventually dropped and the show became known as The Grim Adventures of Billy and Mandy.