She was a former antagonist She is a fun-loving and mischievous 1,000-year-old vampire Queen. Marceline plays an electric bass, which she made from her family's heirloom battle-axe as revealed in "It Came from the Nightosphere", and occasionally plays musically with Finn and Jake. Unlike most vampires, she reveals that it's the color red she likes to suck out of various things, and blood is only appealing because it is red. Like a traditional vampire, she is vulnerable to sunlight and is capable of turning into a bat, varying in size. The show generally portrays her as a close friend of Finn and Jake, but she often feels alone as depicted in "Marceline's Closet". She is Princess Bubblegum's girlfriend.
Marceline is over a thousand years old, as she said in "Evicted!" and "Daddy's Little Monster." In "Memory of a Memory," one of Marceline's memories shows her as a young girl playing near the burning ruins of a city, presumably during the Mushroom War. Later on in "I Remember You," a flashback shows the Ice King giving Marceline a doll, which is eventually named Hambo, to comfort her during the war. In "Finn the Human," it is confirmed that Marceline met the Ice King before the war as they were together before the detonation of the Mushroom Bomb.
As a teenager, Marceline's relationship with her father was damaged when he ate her fries in the ruins of a diner. She later sang about this event in her "Fry Song." The first memory where she is chronologically seen as a vampire is when she moved into the Tree Fort (that would later become Finn and Jake's home) with her boyfriend Ash. She would later dump him for selling Hambo as it was her most prized possession. Marceline has also mentioned her mother, but no in depth information has been given about her or their relationship yet.
In "Evicted!" Marceline says that she has been all over the Land of Ooo, claiming that she has ridden a Giant Goldfish and traveled to the Fire Kingdom. However, according to Natasha Allegri, she was lying and trying to show off. She has also apparently gained several places of residence throughout her life, which may have been in her attempts to get away from the Ice King.
It's unknown exactly when or how Marceline became a vampire, although she did not have a bite mark on her neck when she was younger. As revealed by her alternate version in Farmworld, Marceline does consider herself half-demon, which may be a major contribution to her longevity. This is further supported by the fact that her Farmworld version lived to be over a thousand years old, despite having never turned into a vampire because she stayed with Simon's corpse. Her demonic origins most likely were the direct cause of her ability to survive. However, this side does not give her any magical powers or abilities such as flight and transformation as well as a similar eternal youth to her father, which she appears to have gained due to her vampire powers, due to the fact that her father seemingly does not possess the abilities she has.
Marceline first appears in "Evicted!" when she arrives in Finn and Jake's tree fort and forces the two to leave. Marceline claims that the place is rightfully hers as she used to live in it. Finn and Jake reluctantly leave when they decide that they are no match for Marceline's powers. After the two finally settle down in a new home, Marceline immediately turns up and shows them that she owns that place as well. She also fights Finn and pretends to kill Jake, but she reveals that is was a trick, and she was really just having fun.
In "Henchman," Marceline forces Finn to be her henchman and Jake tries to save him. She tricks Finn into doing things that seem wrong. However, she is just having fun with him and their deeds were actually harmless or even good. Near the conclusion of the episode, Jake knocks away Marceline's parasol and attempts to kill her while she is in pain due to the sunlight. Finn tries to convince him that she is a good person who just likes to "play games" and make things seem worse than they are. Eventually, Jake leaves after Finn deceives him into thinking that Marceline was dead. However, Finn had secretly helped Marceline survive the attack by letting her hide in his backpack.
In "It Came from the Nightosphere," Finn helps Marceline record a mournful song she wrote that questions her father's love for eating fries that she had bought in the past. Marceline reveals the only way to see her father is to perform a ritual to open a portal to the Nightosphere. Finn immediately follows her instructions and allows her dad, Hunson Abadeer, to enter Ooo. Marceline is angry at Finn for releasing her father and is upset even further when Hunson Abadeer steals her ax bass. Finn immediately sets off to stop the Lord of Evil from causing chaos in Ooo, and Marceline comes along so she could take the bass back from her dad. Finn eventually manages to get Hunson Abadeer to start talking things over with his daughter and uses this distraction to send him back to the Nightosphere. Although upset that Finn interrupted the two right when they were beginning to make up, Marceline is relieved that the drama is over.
In "Go With Me," Marceline gives advice to Finn to help him attract Princess Bubblegum's attention so he could attend Couples Night with her. Marceline gives Finn various ideas and advice, but they all backfire and gets him kicked out of the Candy Kingdom. Later, Finn asks Marceline to go to the movies with him, but she misunderstands this as a date proposal and responds angrily. However, she accepts Finn's offer after he explains that he merely asked her as a friend. At Couples Night, the two find the film disgusting and crash the event before riding off with her on wolves.
In "Heat Signature," Marceline reveals more of her condescendingly playful nature when she and her ghost friends trick Finn and Jake into believing they have become vampires. However, Marceline's friends go too far, putting Finn and Jake's lives in danger, and she ends up saving them from the ghosts and revealing to them that she was tricking them all along.
In "Memory of a Memory," Marceline is put under a sleeping spell by her ex-boyfriend Ash. When Finn and Jake explore her memory to wake her up, bits of her past are shown, from a moment when she plays with Hambo alone in the ruins of a city, to her tearful fry incident with her dad, to her angry breakup with Ash after he sold Hambo to a witch. After Finn and Jake manage to wake Marceline and remind her of why she broke up with Ash, Marceline lets Finn help her beat up her ex-boyfriend.
In "What Was Missing," it is evident that Marceline and Princess Bubblegum care for each other to an extent, despite ending their relationship on seemingly bad terms. Marceline sings I'm Just Your Problem out of frustration, confessing that she does not feel like she is enough for Princess Bubblegum, but wants to make up with her regardless. At the end of the episode, it is revealed that Princess Bubblegum's most prized possession is Marceline's old rock shirt.
In "Marceline's Closet," Finn and Jake hide in Marceline's house and end up hearing her recording some of her deepest secrets in song. They are caught when they try to escape her house while she goes to bed. She surprises them by saying she does not mind that they hid in her house, and explains that she hides at their place "all the time."
In "Return to the Nightosphere" and "Daddy's Little Monster," Marceline invites Finn and Jake to visit the Nightosphere, but is bothered by her dad, who wants her to follow in his footsteps and take his place its ruler. She refuses, and sings "Not Just Your Little Girl" with Finn and Jake, but is transformed into demon form by the Nightosphere amulet her dad gave her. Finn and Jake eventually manage to remove the amulet from Marceline's neck and when they are about to escape the Nightosphere again, her dad runs up and tells her he is proud of her anyway.
In "I Remember You," its revealed that Marceline and Ice King were friends during the Mushroom War, while the wizard was still Simon Petrikov. Ice King drops in on Marceline at her house uninvited for a jam session, bringing with him some old papers and memorabilia as inspiration for songs. When Marceline recognizes old pictures of her and Simon and some of Simon's writing, she tries to jog Ice King's memory, but it is no use. Marceline gives up trying to reason with Ice King, who cannot seem to remember any of his past. They sing an emotional song together, using a note that Simon wrote to Marceline long ago as lyrics. Marceline ends up in tears as she sings. Simon wrote the note as an apology to Marceline for when he would not remember her.
In "Finn the Human" and "Jake the Dog," it is shown what Marceline would be like if she had not become a vampire, and remained as a half-demon. In the Farmworld timeline, she is an overweight, crazy old lady. She protected Simon's body (as he died since he never became the Ice King), and tries to warn Finn about the crown before she is killed by the Mushroom Bomb. Jake, however, undoes Finn's and the Lich's wishes, and the world returns to normal.
In "Bad Little Boy," Marceline overhears Ice King's stories about "Fionna and Cake" and tells him and his previously kidnapped princesses a story of her own. This version introduces her fictional male counterpart, "Marshall Lee."
In "Simon & Marcy," Marceline tells Finn, Jake, and the Ice King about some of her and Ice King's adventures almost 1000 years ago while he was still Simon Petrikov. Her tale is set after the Great Mushroom War, and little Marcy ends up getting sick. Simon goes to great lengths trying to make her better while protecting her from the dangers around them. She tries to discourage Simon from using the crown for its powers due to its negative effect on his sanity. Marceline recovers after Simon finds some chicken soup for her, but he ends up calling her "Gunter" instead of "Marcy."
In "Sky Witch," Marceline enlists Princess Bubblegum's help to find Maja the Sky Witch, so that she can get Hambo back. After Bubblegum gets them past Maja's outer barrier, Marceline fights Maja's Crabbit Familiar, while Princess Bubblegum attempts to look for Hambo. Princess Bubblegum eventually encounters Maja, and they make a trade: Princess Bubblegum gives the witch the rock shirt Marceline gave her, while Maja gives Princess Bubblegum Hambo. Maja is satisfied with the trade, explaining that the prized shirt holds far more "sentimental freshness" and "psychic resonance" than Hambo does. Marceline and Hambo are reunited.
In "Red Starved," Marceline, Finn, and Jake, go to an ancient ruin to find the Spoon of Prosperity. After Jake eats the red erasers Marceline brought for food, she reveals that if she goes too long without feeding, she will lose control and would kill anyone around her. Knowing this, Jake ties her up, but she later escapes. Fortunately, Princess Bubblegum bursts through the wall in her Sand Worm just before Marceline is about to kill Jake. Marceline drinks all of the pink ("low-grade red") from her, but Princess Bubblegum puts the spoon on her nose and returns to normal.
In "Betty," Marceline, Finn, and Jake are in the middle of a jam session when she receives a call from Simon, who asks her for help finding Betty and tells her to bring Hambo. When she arrives at the Ice Kingdom, she is very happy to be reunited with her old friend.
In "Ocarina" she had a very quick cameo when Kim Kil Whan mentioned how he received the deed to the tree fort. He said he got it from its previous owner, Marceline the Vampire Queen, in exchange for the lunatic bass. He projected an image of her holding the new bass with his horn, then made it disappear.
In "Princess Day," Marceline and Lumpy Space Princess decide team up to get even with Breakfast Princess, who called Lumpy Space Princess a "pamplemousse." Throughout the episode, Marceline appears to encourage Lumpy Space Princess's bad behavior.
- Levitation: She has the ability to float in the air and fly with controlled speed. While sleeping, her body floats in place.
- Pyrokinesis: She can light fire with only her mind, seen in the episode "Evicted!"
- Invisibility: In the episodes "Heat Signature", "Bad Little Boy", and "Red Starved" she can turn invisible at will.
- Necromancy: In the episode "Henchman" she can raise the dead.
- Rapid Healing: In several episodes, it is shown that she can heal quickly, like other vampires.
- Shape-shifting: Marceline has the ability to turn into numerous forms, such as a bat, wolf, lizard, gargoyle, and a large tentacle monster. Her clothing can either be preserved while she is in another form, or it will disappear until she returns to normal.
- Eating shades of red: Unlike traditional vampires, Marceline is able to sustain herself by simply eating the color red from things like strawberries and clothing. This turns the objects a white/grey color. In "Evicted!" she says she "sometimes" eats blood, but she has never been seen doing it yet. Another thing to note is she says she gets lucid dreams when she eats tomatoes.
- Other abilities: She is a expert bass player, proficient singer, and can play basketball.
- Olivia Olson who also plays Vanessa Doofenshmirtz from, Phineas and Ferb. Both of them, have to deal with their evil father's side.
- In many fanfictions, many Phineas and Ferb fans love both of them to meet each other in real life and become friends.
- Vanessa and Marceline has the same color, Vanessa's clothes is black while Marceline has a black long-hair.
- Marceline is possible a vegan, as she is never seen in the series doing it. Unless; a probability that not sucking blood might be being as a vegetarian.
- Her relationship with Princess Bubblegum, is supposedly be a romantic idea. But instead; they became instant friends
- Theoretically speaking, Marceline's mother might be alive.
- However; this sentence/statement is crossed out.
- Marceline is 1003 year old laid-back teen, who overcomes and has a fear on growing up as a teen.
- In the human version, she is obviously 13 year-old. (While in the vampire age, she is 1003 years old).
- It is confirmed that she had a homosexual relationship with her ex-girlfriend Princess Bubblegum, until they get back together in the series finale.