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

Gerard is a lieutenant aboard the Sutherland. Crystal is Sutherland's warranted master. Both of these men served with Hornblower aboard the Lydia, and he holds them in high regard. Gerard and Crystal are described in vague terms. All we learn is that both men are of average height and build. Gerard takes great pleasure in quick, accurate shooting. Crystal is an exceptional sailor.