Mr. Grimm is the father of Milton Grimm and Giles Grimm. He made his appearances in the flashbacks on Thronecoming (TV special). He is one of the previous Brothers Grimm, he is also responsible for collecting and organizing all fairytales.
In the flashback he was a loving father to his children and love telling stories he also collected a bunch of fairy tales from his story.
He was shown wearing a green coat and black pants with black shoes and a white button t-shirt in his formal clothes.
He makes his appearances in a flashback when Giles Grimm was looking at his picture of his brother when he what's going back into the house to learn his father what happened to his brother. Thronecoming (TV special)