"Topknot" was the nickname Missy gave to a patient onboard a Mondasian colony ship.
Originally part of the patient expedition sent from the hospital on Floor 1056 of the ship to capture the humans of Floor 0507 for cyber-conversion, Topknot was, like all the other patients, shot by the adult farmers at night, and chained to a crucifix in the fields of the solar farm during the day. The night after a shuttlecraft crashed on Floor 0507 from Floor 1056, containing the Twelfth Doctor, a cyber-converted Bill Potts, Nardole, Missy and the Saxon Master, Alit followed the Master and Missy to the crashed shuttlecraft, but was ambushed by Topknot; before the patient could grab Alit, the young girl followed the Master and Missy down into the superstructure walkways underneath Floor 0507.
Eventually, Topknot caught up to, and attempted to attack, Alit and the two Masters. However. Missy was able to parry Topknot with her sonic umbrella long enough for the Master to disable Topknot and reprogram the patient to obey his voice commands, a function he had programmed into all of the patients. After operating the controls for a service-cradle nearby, Topknot joined the Master, Missy and Alit on their journey to Floor 0508. Whilst riding on various pieces of automated farming equipment, Topknot injured one of his legs after falling out of a farming machine when trying to get off it. The Master eventually used Topknot's scanning capabilities to locate a means of hacking into the patients of Floor 1056, and the group rode on several pieces of farming equipment to a huge network of storage silos.
Upon arriving at the storage silos, the group encountered a patrol of CyberNeomorphs, although Topknot remained hidden from the Cybermen. After the Master was rendered unconscious by the Cyber-Leader and trapped in a storage cell by the lifts to Floor 1056, Missy and Alit escaped the Cybermen and found Topknot, who was ordered by Missy to remain at the bottom of a weather control centre while Missy and Alit rode a lift to the top of the centre. Later, as part of her plan to rescue the Master, Missy sacrificed Topknot by using him as a distraction for the Cybermen while she freed her previous incarnation from his storage cell. (PROSE: Alit in Underland)