Fou is one of the main characters of Fate/Grand Order, acting as a mascot of sorts. It's a squirrel-like creature, "a cute animal" that appears to be able to freely walk around the Chaldea Security Organization. It is met by Fujimaru while it's alongside Mash and accompanies the two in their travels to different Singularities.

Spoiler

Fou is Merlin's familiar and a creature belonging to the Beast class, holding the title of Beast IV