Who is Captain Thumbull from Under the Jolly Roger: Being an Account of the Further Nautical Adventures of Jacky Faber and what is their importance?

Captain Thumbull takes over command of the Wolverine when Jacky is relieved. He is also the man who orders the Emerald destroyed after Jacky's Letter of Marque is revoked and her crew is taken. He is also the person who sets Jacky free, saying he'll report she'd escaped during the heat of battle. He refers to her as "Lieutenant," a sign of his respect for her.