Grim is over one hundred and thirty-seven thousand years old (as his early childhood took place during the Stone Age) and speaks with a Jamaican accent. The continuity of how Grim got his reaper status and tremendously strong and powerful supernatural powers comes up quite a few times and it is unknown which way he really got his supernatural powers (for example, in The Wrath of the Spider Queen movie, he was elected to his position as the Grim Reaper while he was in middle school; however, in A Grim Prophecy, it is shown that he was the Grim Reaper since his childhood with his parents forcing him to be the Reaper, which is further contradicted in a later episode where he is seen stumbling over his scythe to become Grim Reaper). His long scythe is the source of all of his supernatural and magical abilities, and possesses many magical capabilities and qualities; although he is still capable of using some incredibly powerful magic spells without it, though these instances are quite rare.
By losing a limbo contest to Billy and Mandy, Grim is fated to be their "best friend" forever. Unfortunately for the Reaper, the contract legally binds him to them, such that (as revealed in the episode "Hurter Monkey") if he ever breaks his friendship with them, he would be forced to spend eternity in Underworld Jail (much to his terror, as he doesn't know what kind of inmates lurk there) as a result of contract violation. Despite this however, he has often tried various ways to get out of this servitude (in the Big Boogey Adventure, Boogey notes this as "invalidating a legal and binding contract"). Though Billy is friendly towards him, Mandy treats him dominantly, and this angers and irritates him to the point where he constantly fantasizes about killing both of them. However, there have been instances that show that he does indeed care for them. He is often forced to do their chores and create supernatural fun for them. Grim is slowly adapting to modern life and is usually seen in his free time watching television (mainly horror movies or soap operas). His adaption to daily life also leads him to neglect his duties as Master of the Underworld and, eventually, not even care about who uses his enormously powerful scythe, seeing as how it is constantly stolen. He also has a bottomless trunk, where all types of dark magical and mystical objects are found; Billy usually abuses them, creating extensive damage, chaos, ruin, death, and destruction.
His friend-and-hate relationship with Billy and Mandy seems to vary from each other. He seems to be more annoyed with Billy than Mandy, due to his idiocy and stupidity. Although Billy abuses Grim's magical and supernatural powers as much as Mandy, Billy treats Grim with much more fairness, generosity, and kindness, resulting in Grim actually caring and liking Billy. In Mandy's case, although they both share the same cold-hearted cynical and misanthropic view of humanity and apathy for the welfare of Billy and other characters, Grim tends to dislike her more than Billy due to her abusive, cruel and disrespectful actions towards him. Like most other characters, Grim greatly fears Mandy, but is able to openly rebel against her when she becomes too obnoxious and abusive.
Grim owns the giant three-headed dog Cerberus but has no control over it whatsoever and is terrified of it. Mandy seems to be the only person capable of taming and commanding it effectively. He is immune to the effects of Horror's Hand as he already lives his greatest fear every day living with Billy and Mandy, but he stated he turned it off upon obtaining it, so there may be something he fears more.
While it is shown that Grim does care for his scythe, he often makes the mistake of letting Billy (and in rare instances Mandy and Irwin) use it, or loses it in other ways. In Billy and Mandy's Jacked-up Halloween, Grim let Billy use his scythe as an accessory for his costume, only to have it stolen by Jack-o-Lantern, who as it turns out is an old enemy of Grim's. Another element to the show is how characters from Grim's past often show up and often take revenge on him for what he had done to them.