Forget everything that you just read about Marceline in the previous comments.
Marceline was born half human half demon. She was turned into a vampire when she was bitten by the Vampire King when she was 18-ish. All the powers that she has now are from when she was a vampire hunter, by being half demon she was able to suck out the powers of the vampire from their soul. You can learn this from the 8 part series in Adventure Time, called Stakes.