Marceline the Vampire Queen (voiced by Olivia Olson) is one of the main characters of the Cartoon Network show Adventure Time (2010-2018). She is a fun-loving vampire queen and musician who plays an electric bass that she made from an heirloom battle-ax.
Marceline was born to an unnamed human mother (voiced by Rebecca Sugar) and the demon Hunson Abadeer. When she was a child, the cataclysmic Mushroom War occurred, and soon after, she developed a father-daughter-like bond with Simon Petrikov, who would one day turn into the Ice King.
She is reunited with her father but eventually leaves him because of his selfish ways. In the following years, Marceline roamed the land of Ooo and began killing vampires, who had taken over much of the land. Marceline soon discovered that, upon killing a vampire, she was able to acquire their unique abilities thanks to her demonic power to absorb souls. However, when she tried to fight the Vampire King, she was bitten and turned into a vampire herself. At some point, she began a romantic relationship with a magician named Ash. The two moved into the Tree Fort, but their relationship would come to an end. This is also when she meets Princess Bubblegum, but Princess Bubblegum pushes Marceline away due to the pressures of running a kingdom.
All of this happened before the events of Adventure Time. Eventually, Bubblegum and Marceline reconcile, and the two grow closer throughout the series.
In the series finale, Marceline sees Princess Bubblegum get crushed and rushes to save her. She confesses that she cares deeply about Princess Bubblegum and the two kiss.
At the end of the last episode, they are seen cuddled up on a couch together, enjoying their life as a couple.