Grace is a pivotal character in the narrative of Goodbye, Lara, serving as the catalyst for the central tragedy. She is the sorceress who, two hundred years before the main story, gave the mermaid princess Lara a forbidden potion to take on human form. This act was driven by her role as a heretical witch, one who defied the laws of the mermaid world to grant Lara's wish for human love. While her initial portrayal fits the archetype of a magical enabler in a fairy tale, her character is suggested to carry significant moral ambiguity, functioning less as a pure antagonist and more as a complex figure representing the price of desire and the temptation of forbidden paths.

Following the tragic outcome of Lara's story, where the mermaid turned into sea foam after failing to find true love, Grace faced severe consequences. She was banished from the mermaid world for her transgressions. Forced to live on land, she now resides in the city of Otsu in Shiga Prefecture, near Lake Biwa, where Lara eventually reappears. In her banishment, she lives in a reduced form, now existing as a fish, and shares a household with a boxing high school student named Mari Otsu. This unique living situation forms her primary relationship in the present timeline, establishing a connection between the ancient, magical world and the modern, everyday setting of the series. Her relationship with Lara is foundational, built on the past transaction of the potion, and she is positioned to play a key role in the present as Lara is reborn and navigates a second chance at life and love.

Grace's motivations are rooted in her willingness to break taboos, suggested by the description of her as a heretical witch, and her decision to assist Lara despite the severe risks. Her role in the story is to serve as the link between the fairy-tale past and the contemporary present, holding knowledge of the original curse and the magical laws that govern the world. Her development is implied to involve a transition from a powerful magical figure to a banished, humbled creature living in the human world, offering a unique perspective on both realms. Her notable abilities include the powerful magic required to brew a potion that could transform a mermaid into a human, a feat considered forbidden and dangerous, as well as the capacity to exist in a fish form, suggesting a mastery of transformation and self-preservation magic. Her presence constantly reminds the other characters of the price of wishes and the enduring consequences of past actions.