Puck is an artificial spirit crafted by Echidna over 400 years ago, initially appearing humanoid before his later transformation. He ranks among the Four Great Spirits under the title "Beast of the End," a position he claimed by defeating the former Great Spirit of Fire, Melakuera. His centuries-long search ended when he found Emilia frozen in Elior Forest. He freed her from an icy imprisonment lasting nearly a century, overcoming ice solidified at absolute zero—a feat demanding power equal to Emilia's own. Following her rescue, Puck became her contracted spirit, mentor, and parental figure, affectionately calling her "Lia" or "my daughter." His contract with Emilia typically limited his physical manifestation to between 9:00 a.m. and 5:00 p.m., though he could draw on her Od (life energy) to appear during emergencies. This arrangement imposed significant mana costs on Emilia, as Puck absorbed mana directly from her instead of the atmosphere, a trait shared only with Beatrice. This contract also sealed both his and Emilia's memories due to a broken vow with Echidna forbidding him from contracting with her, a seal maintained until the contract ended. Puck generally displays a cheerful, playful demeanor, especially around Emilia, Subaru, Beatrice, and Roswaal Manor's residents. He possesses empathic abilities, reading surface-level thoughts and intentions to form immediate trust, as he did with Subaru. However, Emilia's death triggers a drastic shift, plunging him into a cold, vengeful state where he disregards all other bonds. In this scenario, he transforms into his "Beast of the End" form—a 20-meter-long, cat-like entity with glowing eyes—and initiates global destruction by draining surrounding mana, causing widespread freezing. His abilities encompass mastery over multiple magic types, with exceptional fire magic proficiency surpassing even Roswaal. He specializes in ice-based attacks, conjuring razor-sharp ice spikes or freezing targets to absolute zero. Other capabilities include casting Yin magic like "Shamak" (sensory deprivation), basic healing magic, planting "Ice Flowers" that bloom from negative emotions, detecting and removing inactive curses, and telepathic communication with spirit-affiliated individuals. He can manipulate his size from his default 9-centimeter form to larger variations, though maintaining the Beast of the End form demands immense mana. During Sanctuary events (Arc 4), Puck forcibly severed his contract with Emilia to lift the memory seal, enabling her to overcome the Witch of Greed's Trials. This act exhausted his power, leading to an extended dormant state within a glintstone worn by Emilia. His absence profoundly impacted Emilia's emotional stability and altered his relationship with Beatrice, who deeply admires him, calls him "brother," and often seeks his companionship. Though inactive, his glintstone symbolizes a potential return, with allies awaiting his eventual awakening. Beyond the main series, the OVA "The Frozen Bond" explores his initial bond with Emilia, emphasizing their mutual reliance. In the OVA "Memory Snow," his playful interactions during a snow festival contrast his typically reserved nature around non-contracted individuals, showcasing his lighter dynamic with Subaru and the Roswaal household. Puck's history includes associations with figures like Hector (the Warlock of Melancholy) and Altair Mathers. He also aided Toska against Silver Dragons alongside Beatrice centuries prior. His reunion with Beatrice at Roswaal Manor after Emilia's unfreezing highlighted their enduring connection, though fragmented memories limited his recall of Echidna or their shared origins.