Commander Gray shows himself only once in the game tutorial. He loses a battle against the Dark Lord alongside Lianna and Colen and then disappears forever.
He has Quintus’ Special Skill “Thunder Ultimatum” and he’s a Legendar/5* Hero and was using similar clothes as Thorne.
Here is some screenshots in case you don’t remeber him:
Commander Gray icon
Commander Gray fullart
Since you have already his artwork done, why not bring him back as an Season 3 hero or even a Hero of The Month? It would be funny.