The narrator tells the reader it's worth saying more about Vere's character, because he plays a major role in the story. Vere was intellectual and loved books, especially history and biography. He had firm convictions that he held on to against "novel opinion." Some officers found Vere odd and anti-social, as he would often speak with obscure allusions. But, the narrator takes care to clarify, this was not out of pretentiousness. It was just Captain Vere's honest display of his noble nature.
The narrator again thinks it's important to know about Vere's true character before learning about his actions in the story. This suggests that people's natures are innate and unchangeable, although other parts of the story may hint otherwise (the fact that the Nore's sailors could mutiny and then become loyal again, for example.) Vere is presented as knowledgeable not out of a desire to appear smart but from true, noble interest.