The Vampire Queen
Marceline is a half-demon vampire, and the eponymous Vampire Queen, although she is at least 1,000 years old, she takes the form of an 18 to 21 year old. In addition, the circumstances surrounding her transformation into a vampire are unknown. In her first role, Marceline functions as the antagonist of the story, forcing Finn and Jake from their home. However, as the series progressed, she became a close friend to the two. Ward has stated that she is not a villain. Marceline's wandering of the Land of Ooo has made her into a fearless daredevil. Her fondness for mischief and fun has turned her into a trickster, although appearing hostile at first, Finn soon realizes that she is "a radical dame who likes to play games". Marceline also possesses several powers, such as levitation, pyrokinesis, psychokinesis, invisibility, self-healing, and the ability to shape shift into various paranormal creatures. Marceline is also a survivor of the Mushroom War. Hints of her past are sprinkled through the series; in "Memory of a Memory" a trip into Marceline's mind shows her as a child wandering around the outskirts of a destroyed city, and "I Remember You" features her meeting Simon Petrikov in a ruined city. She was very emotionally attached to her teddy bear Hambo, which was given to her by a still-human Simon Petrikov, as revealed in the episode "I Remember You". In "Memory of a Memory", after Ash sold the teddy bear to a witch, Marceline was furious and broke up with him. In the episode "Sky Witch", Marceline enlists Princess Bubblegum's help, and they retrieve the bear. Although she is very independent, her detachment often makes her feel alone, as seen in "Marceline's Closet"; while Jake and Finn are hiding in her closet, she expresses most of her feeling via the writings of her 500 year-old diary. Throughout the series, Marceline is seen playing music; in several official sources, she is described as "a wild rocker girl". Several times throughout the series, she jams with Finn and Jake, and various others depending on the circumstances. She is usually shown playing her axe bass, which was formerly her family's heirloom battle axe. According to Ward, she is ambiguous, which explains her ability to play bass with both hands. She often finds it easier to express her feelings through music, as seen with songs like the "Fry Song", about her father, and "I'm Just Your Problem", about her relationship with Princess Bubblegum.
Marceline is over a thousand years old, as she said in "Evicted!" and "Daddys 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 bear, 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.