Elementals

The Elementals are the embodiments of the four main elements that make up the world: ice, fire, candy, slime and the anti-element being lumps. They have existed since the beginning of life, living, dying and reincarnating for eons and millennia. The current Elementals include Patience St. Pim (an Ice Paraelemental from the past that froze herself to survive through the Mushroom War), Flame Princess, Princess Bubblegum and Slime Princess. The current Lumps Antielemental is Lumpy Space Princess.

History
The Elementals are introduced in the episode "Evergreen". Back then, the four Elementals were Urgence Evergreen (Ice), Chatsberry (Candy), Balthus (Fire) and Slimy D (Slime). One day, they met to talk about an incoming green meteor set to hit Earth and filled with evil magic. The other Elementals do not worry about it as the elements they represent would continue to exist and new elementals would arise, but Evergreen insists that they must act by creating a magical crown that will grant the wearer's innermost wish. Evergreen decides he must wear the crown, supposedly because his deepest wish is to destroy the comet. The other Elementals are against this, as they believe that the crown will corrupt him. Evergreen ends up freezing the other elementals to prevent their interference. Evergreen becomes trapped, and therefore instruct his apprentice Gunther to use it instead, but because Gunther's only wish was to be like Evergreen, he became pig headed, arrogant and bossy in the same way. The comet hits anyway, and the Elementals are killed.

In "Elemental", an Ice Paraelemental named "Patience St. Pim" is discovered by Finn, Jake and Ice King, frozen in a boat below Ice King's castle. She freezes Finn and Jake and instructs Ice King to reunite Flame Princess, Princess Bubblegum and Slime Princess. Then, she revealed to them that they are the current incarnations of the other three Elementals. Patience was one of the elementals before the mushroom war, and along with the others had visions of an impending apocalypse. Much like Evergreen before, Patience refused to accept their fate and attempted to convince the others to let her freeze them and survive. When the others refused, Patience froze herself, indeed surviving while the other Elementals died. Patience shows the new Elementals their potential powers, and attempts to make them unite, but the three Princesses show no interest. The Princesses then use their newly discovered powers to escape when she becomes pushy. Patience is freed by the Ice King and insists that she will make her presence known on Ooo.

In the Elements miniseries, Patience manages to unlock the full potential of the Elementals with a magical spell. This works too well however, not just awakening the elemental powers but causing the princesses to be consumed by them. Bubblegum becomes a giant tower of gum that assimilates everything into Candy with a saccharine disposition; Patience becomes depressed and unmotivated by her failure in a dark region of ice; Slime Princess becomes a giant partying blob who assimilates other slime people into her body and Flame Princess becomes a huge dragon that desires nothing but violence. The Elementals divide the world into four regions, one for each, and transform all the inhabitants into elemental versions with distorted personalities. After obtaining the magical jewels that belong to the princesses, Finn manages to reverse the spell with the help of Lumpy Space Princess, the newly discovered Elemental of Lumps (the Anti-Element), who used her powers to restore elemental balance and turn everyone to their true selves.

Powers
Each Elemental has the ability to generate and manipulate their respective element. However, their powers are limited by how much magic is in the world; before the Mushroom War, the Elementals were very weak in the almost completely unmagical world as opposed to modern Ooo. At their greatest, the Elementals transform the world around them into more of their element, including the people, who's personalities become corrupted by the element that absorbs them; Candy people being creeply happy and nice; Ice people being moody, despondent and unmotivated; Slime people being party focused and eager to assimilate into Slime Elemental, and the Flame people becoming violent and crazy and constantly attacking each other or anything else they see. The Lumps Elemental seems to be very self involved, desiring recognition for who they are and for everything to be 'real' and how it should be (in their view.)
 * The Ice Paraelementals have the power to control and create ice.
 * The Candy Elementals have the power to control and create candy (mainly, jelly beans).
 * The Fire Elementals have the power to control and create fire.
 * The Slime Elementals have the power to control and create slime.
 * The Lumps Antielementals have the power to balance the other elementals and restore the natural order.

Trivia

 * While Flame Princess was aware of her elemental powers from her childhood, Princess Bubblegum and Slime Princess did not uncover their powers until Patience revealed them.
 * Ice Paraelementals seem concerned with avoiding their death at any cost, while the other elementals are accepting of it as their elements will not die with them.
 * Each Element seems associated with one of the four classical personality types, with Candy being Sanguine (social and energetic) Ice being Melancholic (quiet and analytical) Fire being Choleric (Aggressive and irritable) and Slime being Phlegmatic (Relaxed and peaceful).
 * It is revealed in the episode "Skyhooks II" that Lumps act as an "antielemental" - a force that maintains the order of reality and restores balance when the elements grow too much.