Mr. Underwood is "the sole owner, editor, and printer" of the town newspaper. The first time we see him is in Chapter 15, when he talks with Atticus during the break at church. Scout describes him thus:
Mr. Underwood had no use for any organization but The Maycomb Tribune, of which he was the sole owner, editor, and printer. His days were spent at his linotype, where he refreshed himself occasionally from an ever-present gallon jug of cherry wine. He rarely gathered news; people brought it to him. It was said that he made up every edition of The Maycomb Tribune out of his own head and wrote it down on the linotype. This was believable. Something must have been up to haul Mr. Underwood out.
So he feels he has the claim on the news, & he doesn't need to go out & find stories-they'll come to him instead. He clearly thinks of himself as important, and the fact that he has come out to meet Atticus suggests the gravity surrounding Tom's trial. Yet he appears again at the end of the chapter, covering Atticus when the mob approaches at the jail. He has his shotgun trained on the crowd the entire time, & he's ready to shoot if someone makes a false move. At the beginning of Chapter 16, Atticus remarks about Mr. Underwood's personality "You know, it's a funny thing about Braxton," said Atticus. "He despises Negroes, won't have one near him." And he still was willing to protect Atticus and Tom from the mob.
Later, after Tom's death, Mr. Underwood appears once more: this time in teh form of an editorial:
Mr. B. B. Underwood was at his most bitter, and he couldn't have cared less who canceled advertising and subscriptions...Mr. Underwood didn't talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children.
Again, we have the theme of "It's a sin to kill a mockingbird" spelled out before our eyes. It seems that even a man who "despises" other ethnicities can have compassion, and can change enough to openly speak out against the death of a man wrongly accused of a crime.