In the novel The Sittaford Mystery, Mr Brian Pearson is one of Captain Trevelyan's nephews, and the younger brother of James Pearson and Sylvia Dering. After the murder of Trevelyan, Brian, as one of four people who stood to benefit significantly from his will, became a suspect. However he was not considered seriously at first because he was believed to be residing in New South Wales, Australia, at the time of the crime.
However, later in the novel, the journalist Charles Enderby found him at Sittaford House having a secret nocturnal meeting with Violet Willett who, with her mother Mrs Willett had rented the house from Captain Trevelyan for the winter. Enderby persuaded Brian to declare himself to the police and from then on, Inspector Narracott began to regard him as a plausible suspect. Subsequent investigations showed that Brian had sailed from Melbourne on a Blue Funnel ship, the Phidias. On board were also a mother and daughter pair, the Johnsons, from Melbourne, who answered to the description of Mrs Willett and her daughter. Brian had arrived in England some two months before the murder and had stayed at the Ormsby Hotel at Russell Square. One day before the murder, he left the hotel for Paddington Station.
The plausibility of Brian as a suspect increased when, under police questioning, Brian refused to account for his movements from Paddington onwards and defied the police to show that he had done anything wrong. Inspector Narracott found him a hot-headed, high-handed man, fully capable of committing the murder, much unlike his brother James Pearson. Brian's explanation for his presence and the secret meeting with Violet Willett was that she was his girlfriend and he was meeting her in secret because her mother may not approve.
Only at the end of the novel does Violet confess the true background of Brian to Emily Trefusis. Brian had met Violet on the Phidias from Melbourne and they had become engaged. Brian had come to Dartmoor and had helped to engineer the escape of the convict Freemantle Freddy from Dartmoor Prison. Freemantle Freddy was, in reality, Violet's father. Although Freemantle Freddy was recaptured shortly thereafter, Violet considered herself happy and lucky because she had Brian.
What happened to Brian in the end is not revealed in the book. Emily Trefusis only wanted to clear her fiance James Pearson of the suspicion of murder, which she had done by the time she heard about Violet and Brian. From her conversation with Violet, Emily sounded sympathetic and it did not appear that she was interested to turn Brian and Violet in to the police. Emily and Violet were, in any case, going to be sisters-in-law.
It is however possible that Inspector Narracott had an inkling of what Brian was up to. At one point in the late part of the story, Narracott asks a subordinate to do some checks on hotels near Princetown, the settlement where the prison was. The subordinate came back to report that the inspector's hunch was right, but it was not "The Duchy at Princetown. It was the hotel at Two Bridges." The context of the following conversation suggests that they had been tracing the whereabouts of some "young man". It is never revealed who or what the inspector was checking up on, but it is plausible that he was following a hunch on where Brian had stayed after coming to Dartmoor and perhaps he suspected that Brian had something to do with Freemantle Freddy's escape. If Narracott ever acted on this hunch, we are not told. Violet never revealed how Brian engineered" Freddy's esape, but certainly, he would have had to stay nearby, and the Two Bridges Hotel (an actual hotel that is still operating today) is only about 2 miles from the prison.