Cath Palug, more commonly known as Fou, originates from Arthurian legend as a member of the Breed of Calamity species embodying the Logos of "Comparison"—the act of envying others' strengths and regretting personal weaknesses. Originally a Beast of the Planet, he served as Merlin's familiar before being expelled from the Isle of Avalon following Merlin's imprisonment. In timelines with an underdeveloped Human Order, he matures into Primate Murder, the pet of Altrouge Brunestud. Within the Fate/Grand Order timeline, however, he is discovered by the Chaldea Security Organization and accompanies Ritsuka Fujimaru and Mash Kyrielight across multiple Singularities and Lostbelts. Fou resembles a small, squirrel-like creature with white fur, frequently noted for a "cute" appearance. His design incorporates purple and pink horns varying in size and color intensity, subtly indicating his latent nature as a Beast of Calamity. He states that without human civilization's influence, he would remain harmless, but unchecked maturation would degenerate him into an "unsightly monstrosity." Personality-wise, Fou harbors intense animosity toward Merlin for his expulsion from Avalon, attacking him on sight. Despite this, he shows deep affection toward Mash Kyrielight and occasionally displays mischievous or suggestive behavior, possibly influenced by Merlin. He identifies himself as the "Beast of Calamity," defined by "ever exceeding the strength of an opponent," growing stronger by feeding on human envy and regret born from conflict. Fou's narrative significance unfolds across key events: Accompanying Chaldea during early Singularities like Fuyuki and North America, Merlin temporarily uses him as a conduit against Cú Chulainn Alter. In Babylonia, Ishtar/Ereshkigal reveals his identity as Beast IV. At Solomon's Temple, after Mash sacrifices herself, Fou locates her soul beyond life and expends centuries of accumulated Magical Energy to resurrect her. This act transfers his "Force of Providence" to Mash, overwriting her three-day lifespan and granting her a normal human lifespan. The cost reduces him to a near-animal state, devoid of sapience and Beast characteristics, implicitly congratulating Chaldea for nonviolently "defeating" Beast IV. Post-resurrection, despite his diminished state, he retains limited abilities: communicating with Cavall II in Shinjuku to help defeat Hessian Lobo and warning Mash in dreams during the Trinity Metatronius arc about an impending Ordeal Call, though Mash forgets this warning and Fou remains aboard the Storm Border instead of rayshifting. In later arcs, he stays at Chaldea during Avalon le Fae while Merlin impersonates him, and disappears in Nahui Mictlan to avoid confronting ORT, as noted by Koyanskaya of Light. Fou's abilities center on his "Comparison" principle, enabling him to grow stronger by absorbing negative human emotions. Under specific conditions, he can perform miracles surpassing True Magic, exemplified by Mash's resurrection in Solomon's extra-temporal Ars Paulina. Post-resurrection, he retains limited capabilities, such as sensing existential threats like U-Olga Marie before advanced systems detect them.