Grim, or "The Grim Reaper," is the embodiment of Death, who was tricked into becoming the friend of Billy and Mandy for eternity after losing in a limbo match over the soul of their hamster. After living with Billy and Mandy for quite some time, he became accustomed to a simple life and began to grow attached to them, although he would never admit it.
In the wake of the Fusion invasion, Grim was able to slip away from both Billy and Mandy to take a vacation in Orchid Bay in the Past.
Unfortunately, Grim's avenue of escape quickly became more trouble than it was worth. Grim has been unable to enjoy his vacation since Fuse's invasion reached the shores of Orchid Bay. His collection of Zuni heads was stolen as a means to create Fusion Grim.
In addition, rumors have been spread about Grim's being a traitor, despite the fact that Grim has donated many "Resurrect 'Em" spots to revive fallen players. These rumors started when the player, acting to help Numbuh One, found a tape of Fusion Mandy encountering him, trying to recruit him to their side, though Grim refused.
In spite of these rumors, Dexter trusts Grim to help him find missing heroes who disappeared during the invasion, curious to see if the heroes have fallen in battle. After having his Zuni heads recovered, and utilizing the "Desk Lamp of Hades," Grim reports that the heroes are not in the land of Hades.
- He's one of the three NPCs who appear in two Fusion Lairs. The others are Blossom and Dexter.
- Grim is one of the two Nanos whose powers are all Group powers, the other being Darwin.
- Grim has opened a chain of soul-replenishing spas called the "Grim Gardens" that apparently became immensely successful, as evidenced by Princess Morbucks's expressed desire to purchase them.