Anyways, I might as well write this surprisingly short story before Marceline and Flame Princess ever actually do meet in Adventure Time.
So one day, Marceline was carrying some grocery bags to her house while whistling a tune. Suddenly, she notices that her door was open and a trail of fire was in the doorway. When she entered the house, she sees—
“Hey! Who are you?!” Marceline asks. Flame Princess was sitting on her couch, Marceline's ax bass held in her hands. “What are you doing with that? Give it to me!”
“Oh, this is yours?” asked Flame Princess handing the guitar to Marcy. “I’m sorry, I didn’t think anyone lived in this house.”
“Why did you stay here?” Marceline interrogated her.
“Well, the place looked nice, so I didn't want to leave—”
“No, I mean, what did you come to my house for?”
Flame Princess didn't answer at first, and then she responded, “Liquid stuff fell from the sky, and it hurt me. I needed a place to go to, so I found this one. I never saw a musical instrument in my life, well, except one time when this dog gave me a serenade, but I wanted to know what it feels like.”
“What, couldn't your parents buy one for you?” Marceline asked. Flame Princess said, “My dad kept me imprisoned for most of my life. He didn't buy me stuff, or let me out. When I did get out, I would touch everything around me, and it would either fascinate me, or burn to flames.”
Marceline realized her dad hadn't treated her as bad as Flame Princess’ did. She looked down at her bass and let Flame Princess hold it.
“Really?” she asked Marcy. “You missed out on life more than I did,” Marceline responded, “and I’m a millennium old. I’m sure it’s all fireproof anyways.”
“But…why?” Flame Princess wondered.
“Because I know something we have in common. Both of our fathers were poopheads to us.”
Flame Princess looked back at the bass and plucked a string from it.
“Yep,” Flame Princess said smiling. “Our dads suck.”
They hugged. Dem feelz.