Emerging from Arthurian legend as Cath Palug, a member of the Breed of Calamity, Fou served as Merlin's familiar within the Garden of Avalon. He observed King Arthur's story through Merlin's records. Witnessing Arthur's demise led Merlin to expel Fou from Avalon, commanding him to seek "that which is beautiful." In timelines dominated by human civilization, Fou manifests as Beast IV, embodying the principle of "Comparison," growing stronger by feeding on human envy, regret, and conflict. Conversely, in worlds with diminished human influence, he becomes Primate Murder, a primordial threat controlled by Altrouge Brunestud. Discovered by the Chaldea Security Organization, Fou earned his nickname and accompanied Ritsuka Fujimaru and Mash Kyrielight across historical eras. Resembling a small, squirrel-like creature, his evolving horn designs subtly hinted at his Beast nature. Fou exhibited a playful demeanor yet harbored fierce animosity toward Merlin, frequently attacking him due to his exile. He formed a protective bond with Mash. His latent abilities included escaping conceptual restraints like the Chains of Heaven and sensing existential threats beyond Chaldea's detection. During the Solomon's Temple singularity, after Mash sacrificed herself protecting Ritsuka from Goetia's Noble Phantasm, Fou located her soul in the void beyond death. Revealing himself as Cath Palug, he expended centuries-worth of accumulated magical energy to resurrect Mash and extend her limited lifespan to that of a normal human. This act sacrificed his sapience and Beast characteristics, reducing him to a seemingly ordinary animal. Residual capabilities surfaced later. In Shinjuku, Fou communicated with Cavall II to strategize against Hessian Lobo. During the "Cosmos in the Lostbelt" arc, he avoided the South American Lostbelt due to ORT's presence and later warned Mash in a dream about an impending ordeal call. Post-resurrection, observers noted signs of his intelligence gradually recovering, described as an "AI-like restart" of cognitive functions. He remained fiercely protective of Chaldea, attacking threats like Koyanskaya of Light on sight. Fou's journey defied his destined role as a Beast of Calamity. Through his bond with Chaldea—especially Mash—he chose compassion over destruction, fundamentally altering his nature without violence.