The character named Pack in Japanese sources is more commonly known in English as Puck. He is a spirit who appears as a small, round cat-like creature with grey fur and large green eyes, usually floating beside the half-elf Emilia. His cute appearance belies his true nature, as he is among the most powerful beings in the story and is counted among the Four Great Spirits as the Beast of the End and the Great Spirit of Fire.

Puck is an artificial spirit created by the Witch of Greed, Echidna, roughly four hundred years before the main events of the story, making him over four hundred years old. He was formerly bound to Emilia through a contract held in a crystal, acting as her constant companion and guardian. His memories of his creation and true purpose are sealed, leaving him with deliberate gaps in his own past.

In personality, Puck is upbeat, playful, warm, and affectionate. He calls Emilia Lia and treats her like a daughter, states that she is the reason for his existence, and generally stays in a good mood when around her and Subaru. He is tolerant of people touching him and seems to encourage being fawned over, even grooming himself during serious conversations. Beneath this cheerful surface, though, he is fiercely protective and capable of destructive rage. His love for Emilia is genuine but unsettlingly single-minded, as he prioritizes her survival above everything else, including her own wishes.

Puck's central motivation is to protect Emilia and keep her happy. To that end, his contract contained a clause: if Emilia dies, he transforms into the Beast of the End and freezes the world. This act is not portrayed as revenge but as a failsafe tied to his nature. In erased timelines, he has destroyed the world in this form and has killed Subaru on multiple occasions.

In the story, Puck plays a major supporting role in the first, third, and fourth arcs and a supporting role in the second. He defends Emilia in battles such as the fight in the loot house, advises her during the royal selection, and provides comic relief through his playful demeanor.

His key relationships center on Emilia, whom he regards as his beloved daughter, and Subaru, whom he treats with friendly teasing, calling him Barusu. He also shares a familial bond with Beatrice, another artificial spirit created by Echidna. Because of his sealed memories, however, he lost much of his past with Beatrice, which left a painful distance between them.

Over the course of the story, Puck undergoes significant change. After the Sanctuary arc, his contract with Emilia is severed and he becomes dormant, forcing Emilia to develop on her own. The story presents this as both a necessary step for her growth and a form of abandonment, ending his constant presence at her side.

In terms of abilities, Puck is a master of ice, able to create storms of ice projectiles or a single enormous attack. More broadly, he can manipulate temperature, able to freeze or heat anything within his domain, with his true element governing temperature itself despite his title as the Great Spirit of Fire. In his base form, his power is limited by the mana supplied through his contract, and he must sleep at night to conserve energy. He is also capable of mana drainage and regeneration. When Emilia dies, he manifests as the Beast of the End, a towering ice creature able to freeze the atmosphere, the land, and everything else, ultimately ending the world.