The series is set in a fictional continent called the "Land of Ooo", in a post-apocalyptic future after a nuclear war called the "Great Mushroom War." References to the war are shown constantly in the series. The nuclear bombs, melee weapons (such as swords and axes), abandoned human items (such as a TV set, headphones, cassette and video tapes) are scattered everywhere, even a skull on the ground. ==
The war's effect on humans is also shown, with there seeming to be no other fully human character except for Finn. However, human remains are scattered all around Ooo.
- Finn Evans – Finn is a 14-year-old human boy who loves nothing more than going to high school and seeing his friends. He wears a hat that covers his extremely long flowing golden hair. He is prone to exclamations and outbursts including "Mathematical!" and "Algebraic!". When Finn was a baby, he did a 'boom boom' leading to his adoption by Jake's parents: Joshua and Margeret, which was detailed in 'Memories of Boom Boom Mountain'. Being so full of energy, he has trouble in situations that require him to do tasks he isn’t strong on. He has some fears, such as the ocean, clowns, spiders, and being judged by older people due to his age. He is the main protagonist of the series.
- Jake Evans – Jake is Finn's first best friend and adoptive brother. He is a 28-year-old (in "magical dog years") shapeshifting dog. His magic powers allow him to stretch or shrink any part of his body to any shape and almost any size, ranging from becoming gigantic to becoming incredibly small. Acting as a confidant and mentor to his energetic brother (though he has a tendency to give somewhat sketchy advice), Jake has a laid-back attitude in most situations, but loves adventure and will eagerly fight when he needs to. His powers help Finn considerably in combat and transportation, but are also sometimes used as nothing more than jovial forms of expression. Jake is in a relationship with Lady Rainicorn. He met Lady in the pilot episode and is quite skilled at playing the viola, which houses a worm called Shelby. He is the deuteragonist of the series.
- Leonard McLean - Leonard is a 17 year old hologram. He's has a great influency and a great taste of English, German, Italian, Latin and French. He's smart, but never good at Biology, due to some issues of a "intestine pulling phobia." Leonard is mostly geeky, kind and well mannered. He can play the guitar in the school band. He is also the deuteragonist of the series.
- Marshalia Abadeer - Marsha is the Marceline's shy and sheltered little sister and Finn's friend since preschool. Marsha is incredibly smart and has a passion for writing. Marsha currently lives with her father and the are hints of a romantic relationship between Finn and herself
- Bonnibel Bubblegum – Bonnibel Bubblegum is a bubble gum humanoid, comparable to Marsha because of her proficiency in science and fluency in Korean and German are a testament to her high intelligence. While Bonnibel Bubblegum is typically kind and well-mannered, her temper can be a powerful force when provoked. Marceline reveals that Bonnibel doesn’t like being called "Princess" She admits that she cares about Finn and gives him a sweater she knitted to keep him warm.
- Lady Rainicorn – Lady Rainicorn is a half-rainbow, half-unicorn creature and one of Jake's class mates. She can turn objects and people different colors. She can fly because her body intercepts light and can dance on it, which also explains her movement and why she has a rainbow pattern. Jake describes her as "the Rowdy Queen" because she can have fun and fight evil. She speaks Korean. In both cases, despite being unable to speak the major language depending on the dubbed versions, she is able to understand it. As the book series goes on, Jake and Lady's relation continue to grow.
- Mr. Petrikov – Mr. Petrikov is the science teacher who is around 2,000 years old. Mr. Petrivov frequently steals young ladies to forcefully marry them.
- Marceline Rose Queen – Marceline is a fun-loving and mischievous teenager. Marceline plays an electric bass, which she made from her family's heirloom battle-axe and occasionally plays musically with Finn and Jake. The series generally portrays her as a close friend of Finn and Jake, but she often feels alone. It was also revealed she lives with her mon and that her dad took Marsha to live with him, however she does not want to admit she misses Marsha. While Marceline has many things in common with Finn, Marceline likes Finn for being 'just' himself, and nothing more or less.
- Leela Space - Leela Space is a snooty rich girl who considers finn and his friends “uncool”. LS is spoiled and sarcastic , constantly seeking to improve her appearance. She is often shown living outdoors with salvaged furniture and appears to be homeless, because she has either run away from home, or is seeking independence from her parents. She speaks with a thick (albeit masculine) Valley Girl Accent. Her parents are rich and possibly love her. It appears that LS wears a tight short-sleeved purple shirt and a thigh length matching skirt with purple pumps. She also chews gum and does her nails during class.
- Fionna Ellis - Fionna is a friend of Finn and is Marsh’s best friend since kindergarden. She and her friends appeared in the book 1 of “Adventure Time High.” She is usually seen in the company of Cake, Bubba, Marshall Lee, and Marsh, her own companions. 'While usually appearing as black dots, her eyes are seen as blue when enlarged. She wears a rabbit-themed hat with exposed locks of blonde hair. Unlike Finn, her neck is shown. Her outfit includes a teal blue shirt with elbow-length sleeves, a dark blue skort, and high-knee socks with two thin, light blue horizontal stripes at the top. Fionna is a heavier set than Finn. She has a green backpack and Mary Jane shoes.
- Cake Ellis is one of Jake the dog’s classmates and jake’s enemy. She has Stretchy Powersand can morph her body just like Jake. Cake travels with Fionna, and Lord Monochromicorn is one of her friends. As a cat, she also possesses the ability to see in the dark. The fur on her tail stands on end or "frizzes out" when there is danger around or when she gets excited.
- Bubba Gumball Bubblegum is Bonnibel Bubblegum’s older brother. Bubba has shown romantic feelings for Marceline and had actually asked her out on a date.
- Lord Monochromicorn is a friend of Cake. He communicates by using his hoof like a telegraph to communicate with Morse Code, rather than by speaking Korean, as Lady Rainicorn does. He stomps his hoof for a dot and scratches the floor like a line for a dash. He is comepletly black with a long gray mane, with white-silt eyes. He is entirely different from Lady Rainicorn's multicolor scheme. He is more horse-like compared to Lady Rainicorn.
- Marshall Lee King is Marceline’s cousin. Marshall Lee has a slim body, has an average height, pointy ears and light blue skin. He initially wears a grey plaid shirt, but now wears a red and grey plaid shirt. He also wears dark blue jeans, and brownish-red with white sneakers.
- Samantha Starfire is one of Finn’s classmates and is Marsha and Marsh’s lab partners. She has long orange-red hair that is made of fire and as such flows above her head. Her skin is yellow-orange and she wears a long, sleeveless dress that touches the ground. She doesn't wear shoes, though her dress usually hides her feet from view. She has two diamond-shaped, red gems; one on her forehead and the other on the front of her dress. The red gem on her dress is shown to be removable as when she has a different outfit as well. She has black eyes that sparkle white and yellow in the light of her fire.
- Victorious Petrikov is Marsha and Marsh’s other lab partner and the daughter of Mr. Petrikov. Although she is the daughter of Mr. Petrikov, she is not treated special. She later on becomes friends with the students and eventually Finn.
- Marsh Abadoe is one of Finn’s classmates and Marshall’s younger brother. Marsh has a slim body, as tall as Marsha, pointy ears and light blue skin. He wears random outfits, like Marsha.
- Ryder White is the owner of the Auto Shop. He is leader of the greaser clique.
- Beemo is a sentient video game console that lives with Finn and Jake. Beemo is genderless.Its appearance contains elements of Vectrex, Game Boy, PocketStation and Atari 2600 game hardware. Beemo also has the features of other household objects, such as a camera and a VHS player and alarm clock in Beemo's "face" (front panel) can be removed in order for its synthesizer to be played manually. The letters on it side spell "BMO", but it has never been stated if this actually stands for anything. Beemo can transport people into a video game by pressing a button on its face. Beemo speaks English with an East Asian accent. Beemo secretly performs human-like actions like brushing its teeth. While Beemo is an object used for recreation on Finn and Jake's parts, it is still considered a close friend and treated as an equal by the two.
- Ricardio was an old kindergarten classmate. He desires for Bubba Bubblegum. Ricardio attempted to get Princess Bubblegum's heart but Finn saved her and Ricardio swore revenge. Ricardio later returned but failed after Marsha beat him in a hand to hand combat battle.
- Tree Trunks is a miniature elephant classmate who speaks with a gentle, Southern accent. Tree Trunks lives in a little home in the forest, surrounded by apple trees which she uses to make apple pies. Tree Trunks is in a relationship with a pig she met.
- Cinnamon Bun is an old classmate and usually tries to impress people by doing tricks or volunteering for tasks, but usually ends up failing. Bonnibel Bubblegum refers to him as being "Half Baked".
- Hunson Abadeer is the father of Marceline and Marsha but only cares for Marsha. He appears first in the chapter "Back to School Night" where Finn finds Marsha with him. Hunson tries to get to know Marsha’s classmates before hearing what other kids say about his relationship with Marsha, managing to help his daughter. However, during the events of "Adventure Time High: Louder Than a Bomb", Hunson later hears that Marsha was feeling a bit alone, realizing his mistake and talking to Finn, who was kissed by Marsha.