His only fault seems Modesty. if he were less modest he would be more agreeable speak louder & look imprudent;— and is not it a fine character, of which modesty is the only defect?— I have no doubt that he will get more lively & more like yourselves as he is more than you;— he will catch your ways if he belongs to you.
In the passage from on making an agreeable marriage what does Jane Austen say is the problem with Mr. J.P. and what does she advise Fanny to do to change Mr. J.P.
I think she's saying that he's too modest, or in other words too soft spoken, quiet, etcetera. Her advice to Fanny is to try dating/marrying/spending more time around him, (the context?) and eventually Fanny's liveliness will rub off onto him.
