Death is the ruler of the Land of the Dead and the Dead Worlds who debuted in the episode "Death in Bloom". In his first appearance, Death plays the double-bass drums in a competition against Finn. He almost kills Finn, but gives back Jake's memory and the life of Princess Bubblegum's plant after Finn mentions Peppermint Butler. Death is shown to have various powers, such as the "Kiss of Death", encasing people in force fields, and sending people back to the Land of Ooo. Years later into the future he is destroyed after being bitten by his son New Death, who took over his job and was soon possessed by The Lich. The role is then held by Mr. Fox.
Death appears to have a friendly relationship with Peppermint Butler as revealed in the episode "Death in Bloom," though it is unknown how long they have been acquainted with one another. It was at the revelation that Finn and Jake knew Peppermint Butler that Death allowed them to return to the surface, even though Finn had lost a music battle to him. According to Kent Osborne, Death had met Peppermint Butler years ago when he started dabbling in the macabre and has been friends with him ever since.
Death seems to have encountered Abraham Lincoln, the King of Mars. In "Sons of Mars," he rejects Lincoln's initial offer of a penny by replying, "Not this time." He seems to have respect for Lincoln's status as a king, referring to him as "Your Majesty."
As a cosmic entity and the overseer of the Land of the Dead and the Dead Worlds, Death has a wide range of incredible powers and has complete control and authority over his realm and the powers within it. In addition to his natural abilities, Death is a skilled drummer and bassist. His genre seems to be death metal, a pun on himself. Some of his abilities include the following:
- Death can teleport at least two people or objects with a red beam that he casts from his eyes.
- Death can travel to and control almost any Dead World, except for the 50th Dead World which requires someone from inside the realm to grant him access.
- Death can create an energy prison to execute anyone who fails to beat him in a music battle.
- In association with the prison, he can create flames from his skull to execute the loser.
- Death can restore memory to those who drank from the "River of Forgetfulness" by the "Kiss of Death," in which he kisses the victim to restore their memory.
- He is able to become intangible to harm and invisible unless he wishes for others to see him.
- Death has absolute control over death and can bring things back from the dead, such as the Princess Plant and Jake.
- He can also take away a person's immortality. He is shown to do this by wobbling his head.
- He is also shown to be able to travel using a manner of flight that resembles driving an invisible vehicle.
- Death lives in a castle of light, implying he might have photokinetic abilities and can control light constructs.
- Although Death is one of the most powerful beings in the universe, there are some characters who can "cheat Death" and thus not be able to be controlled by him, such as the Lich, Orgalorg and Hunson Abadeer. Marceline referred to this as being "deathless."
- It is further revealed in "Wake Up" that even Death fears the Lich.
- Death is right-handed.
- When Death plays bass, he plays a left-handed bass restrung for a right-handed individual.
- In the Adventure Time/Regular Show Comic Issue 3, it reveals that Death from Regular Show and Death from Adventure Time are cousins.
- This is heavily unlikely to be considered canon, however.
- Death was originally going to be some sort of Japanese cosmic alien, according to Jesse Moynihan's blog. This is why Death Castle resembles a large Japanese rock garden. Moynihan later changed it to a design inspired by Alejandro Jodorowsky's The Holy Mountain. Afterward, he changed the character's head into a horse skull because he got self-conscious about the similarity.
- Death looks similar to a horse skeleton, possibly referencing the horseman of Death, rider of the Pale Horse, from the book of Revelation.
- His head also resembles a sun-bleached steer skull (albeit missing horns), a common motif in 20th century Western American art. This relates to his Old West style outfit and the desert-like nature of the Land of the Dead.
- Death shares a couple of similarities with the Venezuelan legend of "El Silbón" (or The Whistler), who is a tall man dressed all in white, wears a hat, and comes only when people are about to die.
- Most versions of El Silbón's origin also involve a son killing his father and then being cursed by his mother, mirroring Death and New Death's fates in Together Again.