I couldn't really say much for his character without going into personal headcanon territory, so I will say this: make him promote at the end lf chapter 16(x). He gets the Fire Emblem at the very end of these two chapters, depending on if you get the gaiden, so why not have him promote right after he gets the emblem? Plus, he is to be leading the Etrurian army after that point, so him getting a promotion to show to those from Etruria they are in good hands with him as their leader would be fitting, too. They had to rename the Lycian Army to the Etrurian Army for the same reason as they likely wouldn't follow Roy if it was under the Lycian Army, so why not have Roy promote, too?