Delia Hartston (known as Delia Morrison since her marriage) is Alexander Roby's former fiancée. Delia was key to the party's investigation throughout the campaign:

• When Dr Bartlett spoke to Roby, the madman initially seemed to think he was talking to Delia
• Grahame Roby expressed a distrust of Delia, suggesting she was in some way involved with The Roby Murders.
• Captain George Withers engaged a private detective to locate her and traced her to Enfield, North of London, where she lives with her husband Peter Morrison. They married less than a year after Delia split with Roby, and he is 12 years her senior.
• Withers and Bartlett visited her. She told them of Roby's dealings with Lawrence Bacon, Malcolm Quarrie and Montague Edwards and also mentioned Coombes. She also provided them with a copy of "The Walker by the Lake" and told them of Clare Melford and the 'nine teeth' that could be found there.
• Withers believed that Delia's husband beat her, and was greatly troubled by this. When Delia invited Withers to her home on 21st November. ostensibly with information about the Roby case, Withers encountered a drunken, bellicose Peter and saw that Delia was sporting a black eye.
• Challenged to fight by Peter Morrison, Withers soundly beat and hospitalised Delia's husband. Delia warned the stricken man to stay away from her.
• After this event, Withers asked his sister Anna to take Delia in and Delia temporarily moved to Anna's house. Dining together at Anna's house shortly afterwards, both Delia and Withers struggled with their growing feelings for one another and the scandal that any relationship would surely cause.
• When Delia received a note from Alexander asking her to meet him at Loch Mullardoch she immediately brought it to Withers.
• After his return from Drakmar, Withers married Delia.