Eunseol, known in the original Japanese version as Koyuki, is a central figure in the world of MÄR, primarily defined by her role as the best friend of the protagonist, Ginta, in their shared hometown of Tokyo, Japan. She is a schoolmate of Ginta and stands out as the only person who genuinely believes in and listens to his stories about a fantastical dream world, which he later discovers to be the reality of MÄR-Heaven. Her faith in him is unwavering, and she is often seen encouraging him, such as when she gives him a chocolate bar and tells him to eat it to become stronger for the sake of that other world. This supportive nature is a core aspect of her personality, as she sees the good traits in Ginta that others overlook, including his passion and imagination, even when his peers mock him for his weak physique and daydreaming.

The background of Eunseol is deeply intertwined with the story's supernatural elements even before her birth. A powerful and wicked witch named Diana, from the parallel world of MÄR-Heaven, used an ancient spell to take a fragment of Eunseol's soul and placed it within a magic stone. Diana then secretly implanted this stone into the sleeping queen of Lestava Castle, who was unknowingly pregnant. The child born from this magical manipulation was Snow, a being who serves as Eunseol's counterpart or magical clone in MÄR-Heaven. This connection creates a psychic link between the two, causing them to share dreams and emotional experiences; for instance, Eunseol begins sweating profusely during Ginta's intense battle with Phantom in the other world, directly mirroring Snow's strain. Eunseol's own early life was touched by tragedy when her childhood friend Kotomi died after collapsing during a game of hide-and-seek, a loss that profoundly affected her and Ginta, who felt guilty for not finding Kotomi in time.

Throughout the story, Eunseol's role is that of an anchor to Ginta's original world and a source of emotional motivation. When a magical gate appears in their classroom and invites Ginta to cross into MÄR-Heaven, she is the one who makes him promise to return one day and share stories of his adventures. After his disappearance, she becomes a pillar of support for Ginta's mother, even cooking for her to try and lift her spirits. Eunseol also reveals to Ginta's mother that she dreams every night of being a princess in MÄR-Heaven, where she sees Ginta, which provides a small measure of comfort and proof that he is alive. Her key relationships are primarily with Ginta, whom she supports unconditionally, and with her counterpart Snow, whose experiences she mysteriously shares. The two of them are nearly identical in both appearance and personality, further emphasizing their unique bond.

Over the course of the narrative, Eunseol's development is subtle but significant. She transitions from a schoolgirl who simply believes in fairy tales to a young woman whose connection to another world becomes an undeniable reality. Her ability to meet Snow in her dreams grows stronger, and she becomes a passive but crucial witness to the events unfolding in MÄR-Heaven, feeling their physical effects. Her motivations are consistently tied to her deep care for Ginta; she wants him to succeed and return safely, and she clings to the truth of his adventures even when no one else will. In terms of notable abilities, Eunseol herself does not possess combat skills or magic. Her primary power is her innate psychic link with Snow, which allows her to share dreams and physical sensations across dimensional boundaries, a direct consequence of Snow being created from her soul fragment. This connection is the most extraordinary aspect of her otherwise normal existence as a middle school student in Tokyo.