He has the appearance of a filthy humanoid with spotted, pale skin, wearing rags, carrying itself like an ape, and has very limited vocabulary, besides clapping, squealing, growling, and grunting.
Ironically, Grunt acts like more of a dog than Addison (an actual dog) does. He responds to Addison's commands as a dog to its master would. When first meeting Addison and Grunt, Grunt was noted to be smiling lopsidedly, As if imagining how our brains might taste as stated by the book. Addison replies later that is how Grunt gives thanks since just earlier, Jacob, with help from Bronwyn, killed a Hollow that was "Most bothersome."