Finn does NOT approve of Marceline's mischievous prankster ways, but he's totally captivated by her butt-kicking appeal. Her cool nerve and aloof jokes about how tubby the Ice King is getting, really strike a chord with Finn's childishness. Finn's heart shrinks a little whenever Marceline talks smack, but he still gets giddy and let's loose a premature teenage bellowing chuckle/scoff.
A masterful fiend of amplified instruments, Marceline is unrestrained super stardom. Marceline spent a few hundred years practicing horns and stringed instruments. What she loves most is setting up her amps and guitar at the edge of the great canyons in the mountains, dangling her feet off the edge and listening to her ballads echo for miles around her. When Marceline has amplifiers strapped to her boots and belt, stand back cause she will actually blow your ear drums out (she is very inconsiderate and cool like this.
Princess Bubblegum shows off her immense wealth and responsibility and Marceline flaunts her freedom and recklessness. They usually blow off this sibling style steam with ridiculous sports and games like Frisbee golf or who can poke a giant six-toed bee's bottom without being eaten.
Sometimes Finn gets trapped in these contests and has to deal with calming down his friends while trying to understand what their deal is. Occasionally Finn becomes the target in these rivalries; for instance: who can kiss Finn on the cheek the most sort of thing... It's terribly embarassing for everyone.
