Cath Palug (キャスパリーグ, Kyasu Parīgu?), also known as Fou-kun (フォウくん, sometimes translated as "Four"?), is a character that appears in Fate/Grand Order.
He is an alternate iteration of Primate Murder -- inhabiting those timelines wherein the Common Sense of Man pulses with life, and the Twenty-Seven Dead Apostle Ancestors do not exist as an organization. He is known as Beast IV.
Cath Palug is a member of a species of organism known as the Breed of Calamity (災害の種, Saigai no Shuu?), which preside over the principle of "Comparison" (比較, Hikaku?)[1]

Originally a Beast of the Planet that became Merlin's familiar, Cath Palug was at some point subsequent to Merlin's sealing driven from the Isle of Avalon.[2] In those worlds where the Common Sense of Man is insufficiently prominent, he came eventually to be known as Primate Murder -- the pet of Altrouge Brunestud.

In the timeline of Fate/Grand Order, he came to reside within the Chaldea Security Organization, journeying across the assorted eras of human history in the company of Ritsuka Fujimaru and Mash Kyrielight.

Cath Palug describes himself as the Beast of Calamity (災厄の獣, Saiyaku no Juu?) that bears the Characteristic (特徴, tokuchou?, lit. "distinct feature") of "ever exceeding the strength of an opponent" (相手より強くなる, aite yori tsuyokunaru?) -- growing unto maturation by the conflict that arises amongst Men, feeding upon their Envies (妬み, Netami?) and Regrets (悔しさ, Kuyashisa?).

Cath Palug is an entity of the Beast class, holding the title of Beast IV.

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Always in my heart, the Dragon of Hololive.