Marnie has long, blonde hair that falls over her back, ending is smooth ringlets. She wears her hair loose for most of her screen time, wearing a blue dress with white frills and a blue ribbon tied at her collar and also wears a green dress with a yellow bow on her collar. She wears at one point a white nightgown with a blue ribbon on her collar and goes barefoot.
At the party, she wears a pink dress with various frills and high heels. Her hair is held in an elegant-yet-simple style, having only one portion held back with a matching ribbon. She sports red earrings.
As an adult and old lady, her clothing is much more simple, having a warm, earthly look.
Marnie was raised in the Marsh House when she was growing up. She was often mistreated by the maids while her parents were out. Eventually, she met a man named Kazuhiko. Kazuhiko and Marnie spent a lot of time together, often sneaking out. One day, Kazuhiko and Marnie went to the silo that Marnie was deeply afraid of, due to the maids' usage of the location to secretly abuse her. She nearly died while in the silo, before Kazuhiko saved her.
After Marnie grew up, she married her childhood sweetheart, Kazuhiko and had one child, a daughter named Emily. While Emily was still a young child, Kazuhiko died of an unknown illness. The shock of his sudden death caused Marnie to fall ill, and subsequently resulted in her having to be institutionalized in a sanitarium for some time. Marnie's parents had since passed away, and Kazuhiko's family was never mentioned, thus, Emily was sent away to boarding school. Years later, Emily (said to be 13 at the time) returned to her mother's care, but had strong feelings of anger towards her due to her perceived abandonment. Later, we see Emily, presumably in her late-teens or early-twenties, arguing with Marnie; their relationship was never salvaged before it was too late: Emily ran away from home to marry the man who had gotten her pregnant. Sometime later, she gave birth to Anna. When Anna was still a baby, Emily and her husband were in a car accident, resulting in both of their deaths. Marnie took Anna in, but died when she was two. Anna was then placed in a foster home, where she was taken on as a foster child, and possibly adopted by Yoriko and her husband.
When Anna was forced to go to the seaside town that Marnie grew up in, she re-lives some of Marnie's memories, with her taking the place of Kazuhiko in (most) of the memories.