Celine is the protagonist of the story, a woman who, after dying from an illness, finds herself reincarnated into the world of a popular novel as the character Celine Eldred. In the original narrative, Celine Eldred is a selfish and unpopular villainess, described as having a plump or overweight physique, who serves as the older sister to the story's heroine. She is a duchess by rank, and her predetermined fate is a miserable death at the hands of the hero prince, Wilfred. The Celine who is reincarnated into this role retains all the knowledge of her past life and of the novel's plot, making her acutely aware of the tragic end that awaits her.

Driven by a strong will to survive, Celine's primary motivation is to change her destiny and achieve a peaceful, happy life. She vows to reform herself completely, embarking on a rigorous journey of self-improvement that includes changing her arrogant personality, committing to a strict diet and exercise plan to lose weight, and dedicating herself to her studies. Her personality is defined by her earnestness and determination; she is not a character who relies on cheat abilities but rather on genuine hard work to transform herself. The voice actress Haruka Shiraishi has described Celine as a kind-hearted person who shows affection to everyone equally, a quality that naturally draws people to her.

Celine's central role in the story is that of a reincarnator fighting against her own narrative death flags. However, her role becomes more complex when her efforts at reformation produce an unexpected and powerful reaction from Prince Wilfred. Instead of avoiding his notice, she finds herself the intense focus of his obsession and doting affection. The prince, who was meant to be her executioner, becomes a yandere character whose love for her is both heavy and consuming. This creates the story's central conflict and dynamic: Celine's desire for a quiet, safe existence versus the prince's overwhelming and possessive romantic interest. Her key relationships are defined by this unpredictable connection with Wilfred, as well as her position as the villainous sister to the original heroine, Adele Eldred.

Throughout the narrative, Celine undergoes significant development. Her initial transformation is a calculated survival strategy, but her actions lead to authentic personal growth. As she interacts with the world and the people around her, including the obsessive prince, her journey shifts from one of simple self-preservation to navigating the complex and often thrilling emotional terrain of being loved too much by a dangerous individual. Her notable abilities are not magical or combative but are rooted in her strength of character. Her most significant ability is her capacity for self-reform and her unwavering determination to change her fate through discipline and a positive outlook, which ultimately becomes the very thing that attracts the final boss prince.