Primate Murder

"For a long time, I always interrupted your conversations, but from now on I'm sure that I can only squeal. Never mind, for it's like a normal day for you guys. Yet, it does sounds lonely. So in the end, I only want to say goodbye to you."

- Fou's last words before he lost his sapience and reduced into a mindless animal (temporarily)

Primate Murder, also known as Fou, Cath Palug, Fourth Beast and the Beast IV, is one of the main characters in Fate/Grand Order and a supporting antagonist in the Final Singular after being revealed to be one of the Evils of Humanity. He is also the pet of Merlin, whom Fou shared a physical resemblence with and strongly hated. He is ultimately defeated in a roundabout way at the end of Observer at Timeless Temple.

He is voiced by Ayako Kawasumi in Japanese version (who is known for voiced all alternate versions of Saber/Altria Pendragon including Saber Alter) and by Abby Trott in English version of - First Order - anime.

Overview
Strictly speaking, Fou isn't the true Primate Murder, but instead an alternate possibility for what the White Beast could have become. Whereas Primate Murder grew into a savage animal specializing in killing humanity, Fou is a cute critter who watches over mankind's fight and eventually gives up his sapience to save a human being. Fou still had the potential to become Primate Murder, but needed a different perspective and a few more centuries to gather power.

Taking the idenity as Fou, the Beast IV seemed to be an animal that appears to be able to freely walk around the Chaldea Security Organization's complex and has a notable attachment to Mash. The protagonist meets it alongside Mash, and it then tends to come along on the various Singularity missions.

He is in actuality a Cath Palug who belongs to Merlin and in other universes where the Human Order was weaker, would've belonged to Altrouge Brunestud, the ninth of the 27 Dead Apostle Ancestors as the Primate Murder, the first of the Dead Apostle Ancestors and ultimate killer of humanity. It is the Fourth Beast and represents the evil of Comparison (as in envying stronger people and regretting for one's own weakness).

By the end of Observer on Timeless Table, initially becoming an enemy after his identity's expose, Fou eventually redeemed himself and sacrificed his own sentinence, to revive Mash in a last ditch of helping the Protagonist into fighting Goetia. After the event, Fou was reduced into merely a mindless animal.

However, during Cosmos of the Lostbelt, Fou shows that he regained sentinence during the chapter of Norse Mythology and managed to have dialogues that could be translated into human language, indicating that he fully recovered his understanding over humanity. Fou is also shown to be in good terms with Ophelia Phamrsolone, as he hugged her according to her request before the final showdown against Surtur. By the time Chaldea arrives at the Wandering Sea, Sion notes that he has some sort of sapience and is still growing.

Trivia

 * In Tsukihime, the White Beast is merely mentioned as being the first of the Twenty-seven Dead Apostle Ancestors with very minimal info on what the beast even is.
 * Fou/Primate Murder is so far the only Beast that has no boss battle after its introduction.