Charles Surface gambles, drinks, and borrows money. He sells a collection of family portraits for profit, much to the horror of his uncle, who was buying the portraits himself under disguise. At the end of the play, when his uncle asks Charles to give an explanation for his behavior, Charles simply states that he cannot. He has no excuses or apologies, for he knows he did a foolish thing and owns up to it. What makes Charles admirable is that despite all of his faults, he has an incredible awareness of self. Charles knows what he is; he knows his reputation for drinking and gambling. He does not attempt to hide this part of himself, which makes him admirable because he is honest. Instead of being ashamed, he pokes fun at himself and this allows the viewers to laugh right along with him. There is a lightness that comes from his honesty that contrasts greatly from his brother Joseph.
Joseph, in contrast to his brother, is incredibly dishonest. Viewers are repeatedly reminded that he is a "man of sentiment," or a man of feeling. However, Joseph is a hyprocrite. He claims to have everyone's best interests in mind but is only looking out for himself. He seduces Lady Teazle in an attempt to get closer to Maria, the girl under her care. Her husband has no idea that this is going on, and sees Joseph as a trustworthy man worthy of Maria's affections. Joseph builds up lie after lie in an attempt to cover up his true intentions. In contrast to his brother Charles, he has an excuse for everything. For example, when Mr. Stanley comes to him for help, he is kind and gentle and says he cannot help, but will do so when he can. Really, he has no intention of helping Mr. Stanley at all, and is not a "man of sentiment" at all.
Furthermore, it's not sentiment that's really important; it's an honest sense of self. Charles becomes admirable because he has a sense of self, and Joseph becomes a villain because he denies and is deceptive about his true nature.