Captain Robert Knowles first appears in Sharpe's Eagle as an inexperienced lieutenant in the Light Company of the South Essex Regiment. He is one of the handful of officers who quietly supports Lieutenant Sharpe against Sir Henry Simmerson, quickly recognizing Sharpe's abilities as a soldier and adopting him as a role model.
Knowles plays a significant role in the recovery of stolen Spanish gold behind enemy lines in Sharpe's Gold and is shortly afterwards appointed as Adjutant to Lt. Colonel William Lawford, the South Essex's senior officer in Sharpe's Escape.
By early 1812, Knowles purchases a captaincy in 45th Regiment of Foot, but is on hand to assist Lawford when the latter is seriously injured during the storming of Ciudad Rodrigo in Sharpe's Company.
Knowles sympathizes with Sharpe when he learns that he has been demoted from captain to lieutenant and is horribly embarrassed when his hero calls him 'sir'. He is one of several of Sharpe's friends who promises to help protect Teresa Moreno who is trapped with her young daughter in the besieged city of Badajoz.
Knowles takes part in a successful escalade of the wall at Badajoz. He then immediately seeks out Teresa, but is shot and killed by Hakeswell at her home.