Cosima Longcaster is the youngest daughter of Dino Longcaster, and the half-sister of Valentine Longcaster and Decima Mullins. Albie Simpson-White describes her to Robin as ‘thin and blonde and good-looking’.  He also calls her a spoiled brat (Chapter 30).

Cosima is of interest to the agency’s investigation into Rupert Fleetwood because she was present at Sacha Legard’s birthday party during which Rupert took the nef.  Sacha tells Strike that she was in tears, and assumes this was because Rupert had upset her (Chapter 36).

Whilst in a hotel room at the Savoy, Strike sees a magazine which features on the cover ‘a very beautiful young blonde’ with the headline ‘COSIMA LONGCASTER – IT GIRL EXTRORDINAIRE’. Inside are four pages of pictures of Cosima modelling diamond jewellery, and ‘something loosely resembling an interview’ (Chapter 74).

Cosima is twenty-one and lives ‘with her parents in a large house in Belgravia, five minutes’ walk from her father’s club, Dino’s’. She has twenty thousand followers on Instagram where she posts at least twice a day.  She has ‘blonde hair, a peaches-and-cream complexion and colt-like legs’ (Chapter 75).

After several attempts to speak to Cosima, Robin eventually catches her getting out of a car outside Dino’s. However, Cosima tells Robin to leave her alone and disappears into the club. Shortly afterwards Dino Longcaster approaches Robin and invites her inside, asking Robin why she is ‘pestering’ his daughter. After speaking to Robin for a while Dino asks one of the waiters to tell ‘Mimi’ to come. Cosima denies crying about Rupert at Sacha Legard’s party, saying that she just felt ‘overwhelmed’ and felt that Rupert turning up might cause a scene. Robin catches Cosima’s eye as she leaves. ‘The girl looked mutinous but also, Robin thought, scared’ (Chapter 96).