Lord Grim

Lord Grim is the hidden main antagonist of Cartoon Network's 2018 film, Adventure Time: The Epic Movie. He is a mysterious, super-powerful, red-skinned, and androgynous devil, who is Me-Mow and Brenda's boss, and the Lich's son.

History
Grim is first seen as the disguise of Billy, was able to secretly lure Finn and Jake into gathering the gems of power needed to activate the Enchiridion. During the escape, one of the Gumball Guards shot at Billy, reveals his true form as Lord Grim, the Lich's son. He angrily reveals his goal to avenge his father's death and ordering all the monsters to kill Finn, but it fails.

Grim learns of Bubblegum's survival and forms an alliance with numerous past villains from throughout the series, including a confused Ice King, in preparation for the final battle upon which Ooo's fate depends: The Gum War. Gumbald crashes the party, with Grim kidnaps Princess Bubblegum and destroys the Candy Kingdom, and creates a new kingdom with an army of undead warriors, and banishes Finn, Jake and his friends out of Ooo, causing Brenda turns to the dark side by dumps Ice King and banishes him to the frozen wasteland.

As Candy Kingdom is now under Grim's control, Lord Grim approached the mystery villain to collect revenge on Princess Bubblegum. Me-Mow and Brenda arrived his master and said what Princess Bubblegum is believed to be bad and Finn not renounces be hero yet, but Lord Grim was never worried and said that Princess Bubblegum protect her new boyfriend and and then he was helping for Bubblegum to not disturb. This plan fails and Bubblegum tears the peace treaty. Back in Gumbaldia, Grim reveals that Lolly cut the lift as part of Gumbald's plan to gain Finn and Jake's trust. Both kingdoms then prepare for war with Gumbald and Lord Grim, teaming up with some of Finn and Jake's old enemies (The Squirrel, Ricardio, Bandit Princess, Samantha, Peace Master, Me-Mow, Pete Sassafras, Ash, Sir Slicer, Scorcher, Fern, and mistakenly Ice King and Gunter).

Later, Lord Grim finally creates a evil plan to take over Candy Kingdom full of undead creatures and monsters, finally summoning GOLB. As the chaos creature attacks, Bubblegum sends her Gumball Guardians to fight it, while Lolly assists with Gumbald's giant robot, and Fern assists using his grass powers. The creature overpowers everyone and seemingly crushes Bubblegum, enraging Marceline, who assumes a more powerful form and kills it. Bubblegum revives, and she and Marceline emotionally express their feelings for one another before kissing, confirming their long hinted romance. The chaos creature then forms again and heads towards the treehouse. Jake witnesses the creature destroying his and Finn's tree house, and becomes despondent. BMO finds him and sings a comforting lullaby to him. The song's harmony negatively affects the chaos creatures, since the Lich's power is derived from chaos itself. Bubblegum encourages everyone else to sing the song, further weakening Grim's monsters.

Meanwhile, Finn, Betty, and Ice King are trapped in a small, shrinking room within GOLB. Grim's influence reverts Ice King to Simon, and Betty to her sanity, while Ice King's crown loses its power. Simon and Betty reconcile, but they and Finn resign themselves to death as the walls close in. Suddenly, BMO's song opens a breach, through which Finn and Simon escape while the heroes fights Grim. Grim angrily betrays Brenda by trying to destroy Candy Kingdom by himself, but Finn stops him and throws a sword at his robot, destroying it.

The fight ends when Betty, however, realizes that the singing has reset the ice crown's power, and so she puts it on wishes for the power to save Simon. This causes her and GOLB to merge into a singular entity, destroying the chaos creatures, summoning a portal that sucks Betty, GOLB, Lord Grim and Me-Mow into it.

Personality
Grim is an evil, greedy, arrogant, cunning, destructive, hateful, manipulative, cruel, and selfish avenger due to his nature. His plan is to kill Princess Bubblegum and destroy the Candy Kingdom to make sure he creates a new kingdom full of undead warriors and also for avenging the defeat of his father, the Lich.

As Billy, he initially appears to be friendly and he was sort of a mentor and fan to Finn, but later reveals his true nature as an untrustworthy person later then it was revealed that he was a facade and disguise to Billy.

Trivia

 * Despite being the Lich's son, Grim was never mentioned in the series.
 * He serves as one of the final antagonists of the series, alongside Uncle Gumbald and GOLB.
 * Despite being the main antagonist, he had became dangerous than Uncle Gumbald.
 * Additionally, Lord Grim is arguably more evil than Uncle Gumbald, even outside of the film as he became more cruel than the other villains. This is because Grim created a new kingdom near Candy Kingdom, and was happy to avenge the Lich, tried to kill Finn and Jake and is downright mad with power.
 * It was thought that Uncle Gumbald, in an attempt to destroy Candy Kingdom, drove the plot.