In the film, he is portrayed by character actor Harry Dean Stanton.
He is very protective of Max and cares for him so much. He got a shotgun so he can protect Max from Killer Kane after he got out of prison on parole. Even though the police tell him that he won't come near his son, Grim doesn't believe any of that, he knows what Killer Kane is capable of and distrusts the lawyers. He is a strong hearted man, but can be kind of cold because he can't always show his true emotions properly.