I think if ralsei really is the shadow of an object rather than an abstract concept then tbh i think he has to be a piece of paper of some kind. The storage closet is strongly associated with paper, with the between-world state seen in chapter 1 just being strewn about papers and castle town's initial state visibly being made of paper.
This said, i do not think that he is a drawing, but rather an initially blank piece of paper shaped into the role of The Prince by the grand fountain and shaped further into Ralsei by the fun gang themselves.
It's notable that ralsei's design changes throughout chapter 1, from the ambiguous (and notably sphere-headed in the frame of the legend where light shines on his face) Prince Of The Dark into a kind of archetypical Final Fantasy Dark Mage and then into Ralsei. And these sprites do not fit into each other! each indicates a *transformation* of The Prince's apparent shape, from a vague outline of a prophecised hero to an RPG party member into the design that toby initially conceived for him.
This, of course, makes sense. The relationship between a darkner's clothes and their body is (as demonstrated by lancer) not so definite as with a lightner. Since darkners are sentient illusions, logically nothing would actually exist under the clothes. A being who exists solely through perception is not going to have more of a body than can be perceived. It then follows that Ralsei's initial cloaked, prophecy shaped form is his initial shape. A blank page, unmarked.
Only through his friendship with the fun gang is he able to form a proper sense of who he is as a person, and thus take on forms that express personhood. If his page was already marked with an image then this surely would not be necessary. He would be more formed but also in turn more static.
This might raise questions of why Ralsei gets to be a character while the rest of the papers in the closet are relegated to scenery, but i think the obvious answer is just that dark fountains are arbitrary about how the shadows of various things are adapted into darkners. The storage closet (containing papers, shelves, dust, mold and a box of chalk) simply does not have enough in it to feasibly use anything else for the role. (well, maybe the chalk, but that would be quite an unwieldy Third Hero!) Looking for a hard logic behind how dark fountains determine what to make into a character darkner and what to make into an object is futile because they operate on narrative and perceptual logic rather than impartial mechanics. (and the apparent distinction between darkners and dark world objects and items is constantly demonstrated to be thin to the point of irrelevance, not just by jevil and spamton but by lancer and rouxls. (it should also be noted how winglade is a weapon in the knight's perception and a foe in susie's perception. a pen is a blade that asserts power on the world to the knight which makes with blackened knife and a source of distress to the teenager who struggles with reading and writing, how interesting!))
The nature of fountains aside, if Ralsei is taken to be a piece of paper, then that adds a whole new dimension to when Susie leaves a mark of her own blood staining his cheek. While the blood stain itself probably won't stay on his character design, this marks the end of chapter 4 as a moment that defines part of who he is forever.
this ramble turned out kinda messy and unstructured but i think i've gotten my point across
