Who is Longley from Ship of the Line and what is their importance?

Longley is a ship's boy (midshipman) aboard the Sutherland. Hornblower described him as "an ugly little fellow, with...stubbly hair and monkey face" He also feels genuine affection for him. Although Hornblower expects Longley to go on and enjoy an illustrious career, the boy is sadly killed during Sutherland's final battle.