Garou makes his first appearance in, "The man who wants to become a monster" chapter. He is the the villain in the "Monster Association Arc". He was a former disciple of Bang and is a menacing character. He wishes to become a monster to fight off heroes, even though he is a regular human being. He picks fights with top heroes and loves fighting to put the heroes in their place. 

For his voice, I was thinking that his voice should sound something along the lines of menacing and evil. However, he should still have a sense of humanity to his voice. He is not entirely evil..... yet.... ( If you read the the Monster Association Arc, you'll see where I am coming from.)