Caster from Fate/Extra Last Encore is not a traditional heroic spirit from human history or myth. Her true identity is Nursery Rhyme, which is the collective embodiment of all children's picture books, particularly those from English literary tradition. She is a conceptual being rather than a proper hero, born from the dreams and wishes of countless children who found comfort in stories. As a Servant, she is a unique existence that reflects the heart of her Master and manifests in a form that mirrors that person's deepest desires and imagination. In the context of Last Encore, she serves as the Floor Master of the third layer of the Moon Cell's Holy Grail War, taking on the appearance of a young girl who is nearly identical to her Master, a girl named Alice, differing only in the color of her clothing. Caster is, in essence, the dream of her Master made real, a reflection of Alice seen through the looking glass.

In terms of personality, Caster embodies a peculiar combination of pure innocence and profound cruelty. She is not malicious in a human sense but instead operates with the simple, unyielding logic of a fairy tale. She treats the world around her as an extension of her own storybook, where games are eternal and the usual rules of life and death do not necessarily apply. She relates to her Master, Alice, as a beloved companion, and her primary desire is to grant Alice's wishes and preserve her happiness, even if doing so requires trapping the girl in an endless loop of a single, cherished day. While kind to those she considers friends, she can be utterly ruthless toward outsiders, viewing them as threats to her sanctuary or as characters who have no role in her narrative. Her personality is colored by a deep-seated sadness, as she is acutely aware that she is not a real person but a fictional doll born from a mirror, and she cannot exist independently of her Master's dreams.

Caster's central motivation is the fulfillment of Alice's wish. Her primary goal is not to win the Holy Grail for herself but to maintain the eternal, happy existence she shares with Alice within the nameless forest. Deeply lonely herself, she clings to the bond with her Master as the only thing that gives her a sense of purpose and reality. She repeats the same day with Alice endlessly, using her power to reset time whenever an undesirable outcome occurs, always seeking the perfect, happy loop. Her desperate wish is to become real and to remain with Alice forever, even as her methods trap them both in a stagnant, timeless prison.

Within the story of Last Encore, Caster acts as the primary antagonist for the protagonist, Hakuno Kishinami, and his Servant, Saber, upon their arrival at the third floor. The environment of the floor is her domain, a large part of which is a manifestation of her Noble Phantasm, the Nameless Forest. She presents a formidable obstacle, not through raw power in a conventional fight, but through her reality-warping abilities. Rather than allowing a final, conclusive battle, she prefers to reset the confrontation every time she or Alice is in danger, endlessly reliving the encounter until the intruders can be eliminated or driven away. Her role is to protect the idyllic yet stagnant world she has created for Alice, making her a tragic adversary who cannot be overcome through sheer force alone.

Her most significant relationship is, of course, with her Master, Alice. The two are inextricably linked, with Caster being a direct manifestation of Alice's subconscious, specifically her loneliness and desire for a playmate. They are symbiotic beings; Alice cannot survive without Caster's power to sustain her in the Moon Cell, and Caster cannot exist without Alice's dreams to give her form. Caster's relationship with the protagonist and Saber is that of a hostile, yet almost playful, adversary. She does not hate them personally but sees them as a disruption to her treasured routine. She toys with them, resets the battle, and tests their resolve much like a character in a story testing the hero, all while hoping they will simply leave her and Alice alone.

Throughout her appearance, Caster herself undergoes little change, as she is trapped in an eternal cycle of repetition. The development instead lies in the revelation of her circumstances to the protagonist. The audience learns that her seemingly cruel method of endlessly resetting time is born from a desperate, child-like love for Alice and a fear of being alone. She is not a monster of pure evil but a broken and lonely Servant trying to preserve the only happiness she has ever known. Ultimately, her fate is tied to Alice's; breaking the cycle their shared delusion forces her to confront the stagnation of their existence, leading to a resolution that is as sorrowful as it is necessary. Her true evolution, as a concept, is her willingness to change from a simple reflection of a wish into a more independent existence, though her story in Last Encore concludes in tragedy.

Caster possesses a number of notable and extraordinary abilities, almost all of which are derived from her nature as a living fairy tale. Her most powerful ability is her Noble Phantasm, Queen's Glass Game: Perpetual Engine Maiden Empire. This ability allows Caster to reverse time by a single turn, effectively resetting the immediate situation to her advantage. In the context of a battle, it allows her to rewind any damage she has taken, making her nearly immortal. More broadly, she uses this power to repeat entire days over and over, ensuring that she and Alice can live in a perfect loop where nothing goes wrong. The memories of these previous loops are accumulated, meaning Caster learns from each repetition while her opponents retain no knowledge of the previous attempts. This Noble Phantasm, however, leaves her momentarily vulnerable when activated, which is the only opportunity to defeat her.

She also commands the Nameless Forest, which is both a Reality Marble and an embodiment of her Territory Creation skill. This is a seemingly endless, sunny plain with a garden at its center, modeled after the Mad Hatter's tea party. Within this forest, anyone other than Alice or Caster will slowly begin to forget everything. First, they forget their name, then their identity, and finally their very existence, causing them to fade away entirely. The only known way to resist this effect is to have one's name written down in advance and read it aloud.

Furthermore, Caster can summon characters from Alice's favorite stories, the most formidable of which is the Jabberwock. This creature is a massive, monstrous beast of immense strength, capable of overwhelming even powerful Servants with brute force. As long as Caster has magical energy to sustain it, the Jabberwock will continuously regenerate from any injury. She also has access to a variety of other magical attacks based on the stories, such as generating tornadoes, creating blasts of ice, and summoning an army of playing card soldiers. Her personal skills include Shapeshifting, which allows her to alter her form as seen in her mirroring of Alice, and Self-Modification, which allows her to connect her own form to her Master's will. Despite these incredible conceptual abilities, her basic combat parameters such as Strength and Endurance are the lowest possible, making her physically frail if her magical defenses are bypassed.