Melanie's character design is a form of foreshadowing itself—her face is obscured by a paper plate in the liminal world. One of the very last memories shown in the first Stairway is Melanie's most horrific childhood memory: shortly after Marin died, her mother kicked her out of the house, making sure to throw plates at her to ensure that she wouldn't return home.
When Melanie reaches Level 36, she unlocks the Broken Plate ability, which increases her Attack stat but decreases her Defense—a reference to the aforementioned memory.
Marin's character design also counts. His face is obscured by a paper boat—an obvious nod to his cause of death.
Additionally, some details on Marin's design that initially appear to be Non-Standard Character Design, such as himonly having one, realistically drawn eye, and his character portrait having no mouth, imply early on in the game that he's been Dead All Along, and the Marin we see in-game is an avatar Melanie created to distract herself from the pain of his death.
It could even be argued that the reason why his design is so unconventional is because he died so long ago that Melanie can barely remember his face, which is why it's incomplete in his portrait.
