Ermine is a female character from Earl and Fairy who stands out for her tall stature and exceptional beauty, though she consistently dresses in men's clothing to maintain freedom of movement during combat. She is the half-sister of Raven, and unlike him, she has notably fair skin. Her appearance includes brown eyes and black hair cut to neck length, and she typically wears a bow tie as part of her masculine attire.

Ermine's background is marked by profound tragedy. Alongside her brother Raven, she was sold into slavery to the organization run by a figure known as the Prince. It was there that she met Edgar J. C. Ashenbert, the man who would become the Blue Knight Earl. She developed deep romantic feelings for him, but her suffering was compounded when the Prince, seeking to torment Edgar, had her assaulted in front of him. This event left her with deep emotional scars that would define much of her later life.

After several years, Ermine managed to escape from the Prince's organization with Edgar and Raven. However, her motivations were conflicted. Fearing that she would become just another ordinary servant once Edgar achieved his desired status as an earl, she made a desperate pact to remain a special presence in his life. As part of this deal, she agreed to act as a spy for the Prince, betraying Edgar to buy herself time and a unique position. Her personality is shaped by this internal conflict; she is intelligent and fiercely determined, but also vulnerable and deeply prideful. Her loyalty is constantly tested, swinging between a desire to protect those close to her and the painful circumstances of her past that force her into questionable actions.

Ermine's role in the story is complex, functioning as both a tragic antagonist and a supportive ally. With the help of Lydia Carlton, she perceived that Edgar was getting closer to obtaining the legendary sword he sought. Overcome with guilt, she confessed her betrayal to Edgar. In an act of sacrifice intended to sever the Prince's hold over him, she threw herself into the sea before she could witness him obtain the sword. She did not die, however. She was later revived by the character Ulysses Barlow, but transformed into a selkie, a seal fairy.

After her resurrection as a selkie, Ermine continued to support Edgar and Lydia from the shadows, though her actions were often ambiguous and contributed to a delicate situation where Edgar was nearly corrupted by the Prince's influence. It was later revealed that much of her spying was done under the orders of Francis, intended to ensure Edgar's survival. Her transformation granted her the magical abilities of a selkie, including the power to survive in the sea and other fairy abilities tied to her magical pelt. She entrusted this pelt, the essence of her life, to Edgar. As Lydia observed, this act meant that Ermine's soul would eternally belong to wherever Edgar was.

Her most notable relationships are multilayered. Her bond with Edgar is one of unrequited love, shared pain, and a lingering, almost spiritual loyalty. Her relationship with her half-brother Raven is shaped by their shared traumatic history and enslavement. Over time, she also develops a bond of trust with the water horse Kelpie, with whom she begins to share a mutual understanding as comrades.

In the later narrative, Ermine plays a key role in the quest to find the Fire Agate. Using her selkie abilities to gather information, she joins forces with the mermaid Rosa and the merrow Jet. During this journey, she struggles with her identity as her heart gradually becomes more like that of a fairy while carrying her pelt. In a poignant turn, she asked Kelpie to let her die as a human, entrusting him with her final wishes. In the anime adaptation, her story is altered significantly; she is already a selkie at the beginning of the series and is with Ulysses, meaning the protagonist Lydia only knows of her by name.