Initially Mr. Otis did not believe in ghosts. Later on, what made him suspect that he had been too dogmatic in the denial of the existence of ghosts? What character trait is highlighted by this suspicion of his?

When the story first begins, Lord Canterville tells Mr. Otis about the presence of Sir Simon's ghost. Mr. Otis flatly denies the possibility as a stereotypical, overly confident American. Mr. Otis claims that if ghosts were real, then they would have already had one on display in a museum or a road show.

"I reckon that if there were such a thing as a ghost in Europe, we'd have it at home in a very short time in one of our public museums, or on the road as a show."

Mr. Otis's opinion of whether or not the ghost is real begins to waver with the persistence of the blood stain. He and his family try everything to get the stain to go away, but the stain comes back each time. Mr. Otis is forced to admit that the presence of the ghost might indeed be a real thing.
Mr. Otis's initial belief that ghosts don't exist stems from his prior experiences. He has never come across a ghost in his real life or come across one on display in America; therefore, his evidence suggests that ghosts are a complete superstition of the British aristocracy.

"But there is no such thing, sir, as a ghost, and I guess the laws of Nature are not going to be suspended for the British aristocracy."

When Mr. Otis is shown evidence that is contrary to his initial thoughts, he shows that he is willing to adjust his beliefs. While Mr. Otis might be a bit rude, the fact that he is willing to admit that he was wrong and change his opinion is a great character trait. The only negative character trait that I feel these scenes indicate is that Mr. Otis must experience something personally in order to believe it. He's a bit of a "doubting Thomas."