Captain Morse is the leader of the Fort Washington prisoners of war whom Isabel agrees to take messages to in exchange for Curzon's humane treatment by his comrades. While the other military officials Isabel deals with, like Bellingham and Colonel Regan, are mostly indifferent to Isabel's position, Morse seems to genuinely treat her with decency and is grateful for her help. He has a good-natured personality, calling Isabel "Just Sal" as a result of her reluctance to give information about herself.
Morse is a strong military leader who desires to help his men and keep them updated about the war even while they are held captive. When he gives Isabel the bread containing the note of Washington's victory at Trenton, he's "ready to jump out of his skin" (39.45) with desperation to lift the soldiers' spirits.
He's also a man of honor and of his word, even using Isabel's services to take a wager to another captain in a lost bet over whether the British would hold their Queen Charlotte Birthday Bash. Generally, he seems like a pretty good guy, and it's a shame all the military guys in this book can't have his attitude of honor and honesty.