The King's request put an immediate bad taste in Desmond's mouth.
He no longer cared about the identity aspect. Considering he was a no name person before he was the duke, many knew that he had a lack of noble background. He was only in the position because of his merits in battle.
His desire to be a Castillian rather than an Eirenguardian lay entirely in the people he surrounded himself with. He wanted the union between he and Leonor to be beneficial to her and to have a husband from a fallen kingdom wouldn't elevate her status as he wanted to. In terms of King Stephano, people wouldn't trust him if they found out Desmond originated in Eirenguard. His name was already unusual enough. The only evidence they lacked was his red eyes, light blonde hair, and pale skin. Luckily, or unluckily, the scarring took care of much of his appearance.