Who is Lady Aster from War and Remembrance and what is their importance?

Lady Aster is a friend of Warren and the man who took over command of the submarine, The Devilfish, when Captain Branch Hoben fails to withstand the stress of battle. Aster is a daring captain but sometimes puts his men at risk. At one point he orders firing on Japanese soldiers who are unarmed and in the water as they tried to escape their sinking ship.