"But there was no mistaking Griff. "That will be enough shouting," he said. A sudden silence fell upon the river. This one will be trouble, Tyrion knew at once."
--ADWD, Tyrion p. 119
As important as "Griff" is in ADWD, I am surprised I haven't seen any portraits of him attempted. I thought I might try one with a quick warm-up sketch.
My thoughts are that Griff should appear as your stern, but cool uncle, but not the boring disciplinarian that Stannis might appear to be. I also added the beard, since it's there when he arrives back in Westeros.
This may end up as a digital portrait at some point, but I am not sure if I want to save this for after I try a quick sketch of "Young Griff" as well.