Gus Upcott is the son of Inigo and Katya Upcott, who is Josh Blay’s personal assistant. Gus is described as a “thin young man” with “painful-looking raised cauliflower-like bumps over both cheeks” and “swelling in one eye” due to his health condition. He has a younger sister called Flavia, who shows an interest in investigative work when Strike and Robin visit the house. The family live on Lisburne Road in Hampstead.

Gus is a student at the Royal College of Music. Although he should be doing his final year there, Katya and Inigo took him out of the college to try and sort out his urticaria, a health condition which severely affects his skin. Katya tells Strike and Robin that Gus also developed angioedema. His throat swelled up, and he was even in hospital for a bit. He sees a specialist dermatologist called Dr Hookham on Harley Street.

When Strike and Robin first visit the house on Lisburne Road, the sounds of an accomplished cello solo plays from Gus’ room, which Robin thinks is so good that it could be a recording. Strike later finds an old YouTube video of Gus playing the cello when he’s trying to investigate Gus’ online activity.

Gus claims he has a girlfriend called Darcy, though his parents believe that they split up. We later find out that Darcy is three years ahead of Gus at the Royal College of Music and had invited him to a party one weekend.

When Robin goes to the toilet at the North Grove Art Collective, she notices a drawing of Gus pinned to the ceiling, which she guesses was drawn by Katya.

In chapter 50, Robin follows Gus from his dermatologist specialist’s building on Harley Street to Fischer’s restaurant, where she watches him on his laptop in the far corner. Anomie isn’t in the game at the time.

Wanting to talk to Inigo for a second time, Robin and Strike drive to Lisburne Road in Strike’s BMW, but on arrival see Inigo and Gus pulling away from the house. They decide to follow them and, an hour later, arrive at the Upcotts’ second house in Whitstable, Kent. Inigo agrees to talk to them for five minutes. He shouts at Gus to go and buy his shopping essentials and, on returning from the shop, Inigo snaps at him to put the food in the fridge, which Gus does, despite him being in a hurry to leave. Afterward, Strike and Robin meet Gus outside, and he tells them that Kea isn’t the only girl his father is talking to –- he is also talking to a girl called Rachel.