SYSA EDITED
He had watched her grow older, while the King stayed eternally young. They all told her to stay away, to beware the beast that lurked inside. No one saw the secret side of him that she did, when they stayed up late at night racing over the hills. Even her beloved Cake hated the monster the night turned him into, which wounded the young adventuress. But Fionna stayed loyal to him, dragged him out with his umbrella during the day and endured the whispers and pointing of the candy citizens.
She fell in love with him at the tender age of 16, and at 17 she was in her prime, yet refused to tell him her true feelings. Deep inside she knew that he was aware of this, a being as old as he wouldn't be that oblivious. His constant flirting only strengthened her feelings for him, her possesevness when his fangirls got too close to him. Of course Marshall notices, but says nothing because he secretly likes it.
They adventure during the night, and slowly he's begun to bring his clothing and stock of apples and strawberries into her house. Cakes notices this of course, but also considers how happy the young girl is with him in the house and says little to nothing. The cat and vampire still disagree on many things, mostly how sheltered Fionna is and that her innocence will get her in trouble. He argues this while she plops herself into his lap and Cake smiles smugly as he blushes and turns away.
They've slayed hundreds of monsters over the years, both have ended on the brink of death. She;s almost confessed to him with watery tears, a gaping hole in her leg. The king had hushed her before flying her to the Prince, panic filling his dead body. It took the kingdom's best surgeons 2 days of straight work to save her leg and another 3 weeks of therapy before she could walk again. Marshall had been riddled with guilt but the headline, "Beast Saves Beauty" made him breathe easy again.
Because this was their love. Forbidden and extremely wrong but made just for them.