Roberts was Second Lieutenant during Hornblower's tour aboard HMS Marguerite, Captain Courtney commanding. He was a white-haired gentleman who seems to have been passed up for promotion to captain.
The story starts with Hornblower as Fourth Lieutenant and after Captain Courtney is wounded in the knee by Fletcher and First Lieutenant Cuffe loses his life in the taking of Castilla as prize, Hornblower becomes First Lieutenant. So, either Peterson or Roberts becomes acting Captain and the other is killed in action.