Billy-Bob is a lawyer in town. His real name is William Roberts Baxter, but he's more casual than that, even in the law office. He used to pal around with Erlo Ackerman and Chick Hannaberry in his younger days, but by the time Tom meets him, he's balding and hunched over from years in the office.
It seems like that famous skepticism toward lawyers is in full effect, because there are hints that other characters do not entirely trust Billy-Bob. For instance, after learning that Tom has been to see Billy-Bob about the cost of taxes on the Breen place, Mr. Hook notes, "I'd have done that for you, Tom, and not charged you a fee, either" (29.7). When Tom says that Billy-Bob didn't mention a fee, Mr. Hook replies, "Well, we'll see what he does. Billy-Bob goes his own way" (29.8). Tom thinks, "He liked Billy-Bob, but the way Mr. Hook had talked had made him uncertain. It bothered him off and on throughout the rest of the fall" (29.13).
But Billy-Bob never does charge a fee (not much of one, anyway), and he's always extremely helpful to Tom. He consoles Tom when the Breen property is sold, helping him to see that he could still buy the barn from the new owner. Tom even grows to trust Billy-Bob enough that he brings the chests he finds on the Breen property to Billy-Bob's in the middle of the night. Billy-Bob helps him protect the money and set up his account at the bank. He also gives Tom a tip on being a good man by indicating that he should set up an account for Birdy for all the help he's given.
Billy-Bob does mention, "maybe as a man of means, later on, you'll have need for a lawyer" (54.37), so there is a lingering touch of the slimy in there. Overall, though, he's very generous toward Tom and gets a kick out of the whole secret-treasure business. Billy-Bob is another male character who is happy to help Tom, and both Tom and Billy-Bob are better for having known each other.