Fionna wasn't some little princess, frail and in need of saving. Marshall enjoyed that. It made her real. Physical. Scarred and marred and his. Part of his world, dark and dangerous, a world of monsters. She was just a righteous one, spending her lifetime to make up for the mistakes of her people, spanning many lifetimes. Marshall saw it as beautiful, as poetic, and his heart twinged at the realization that half of him shared that burden.
So, for as long as she lived, he endured to fight with her, in day or night, and lay with her when she lay. To fall when she fell.