Not knowing much about the show, here's how Bryan describes the character:
Marceline 01st appears in the series as a seemingly cool vampire chick, but then kicks Jake and Finn out of their tree house because it's HERS! Jake is all scared of vampires, but Finn gathers the courage to fight her in the end and become friends! Marceline is a wanderer. She travels all over with her guitar and bag looking for adventures. Does Finn have a crush on Marceline? Go ahead and make her to add to your Tree House set... but protect ya neck!