"I don't think there's any fictional character I hate more than Forrest Byrnes" - Henry (determinant)Forrest Byrnes is a fictional character within the world of Firewatch, depicted on a two dimensional wooden cutout behind the outhouse at Two Forks Lookout.
He is a character created by The Forest Service as a mascot, maybe to appeal to children, however many people, both players and characters in the game, react negatively, finding his appearance unpleasant, including Henry.
He is depicted using only the colour yellow, making him appear even more cartoonish, however his eyes are completely black, something that lends to his unpleasant appearance, Most likely to be that smile.
His name is likely a pun for "Forrest Burns."
He is also depicted carrying a shovel.