Harriwell is an inmate of St Agnes Asylum, and lives in the cell next door to Alexander Roby. He is a brain-damaged lunatic who is likely to be institutionalised until his death.

Harriwell was a witness to, and possible suspect in, the brutal murder of Cuthbert Yates, who was killed in Harriwell's cell on November 27th 1927. Harriwell claims the killer was 'the Devil'. The asylum staff insist Harriwell is incapable of violence.

After the murder of Thomas Clarke, Harriwell approached Dr. Bartlett with a bloody knife, blaming 'the devil' for the murder. Doctor Charles Highsmith rendered Harriwell unconscious with a fire extinguisher, and Harriwell has been arrested for all 3 St Agnes Asylum murders, despite his status as legally incompetent to stand trial.

His most likely fate was to be incarceration at the Broadmoor Hospital for the Criminally Insane.

After Dr. Bartlett testified that he had been attacked by Alexander Roby in Scotland, and Montague Edwards was identified as the true identity of Nurse Evans, Harriwell was moved back to his old cell to live out his days in familiar surroundings.