Harrowhark Nonagesimus is the protagonist of Harrow the Ninth and a returning deuteragonist from Gideon the Ninth. She is 17 years old in Gideon and turns 18 during Harrow. Harrow is the heir apparent to the Ninth House of the Nine Houses. As the Reverend Daughter, Harrow believes it is her solemn duty to shepherd her House, which is currently dying out. Harrow is a genius with bone magic, suspicious of others to the point of paranoia, and stubbornly proud in her ability to outwit and outmaneuver others, which often leads to her downfall.

Harrow shows Harrow as the exact opposite of her projected image; Harrow is a backseat viewer of the events unfolding around her, powerless to interact with them, change them, or understand the political chess maneuvers unfolding aboard the Mithraeum. Harrow’s brain-altering procedures have left her questioning her own reality and unable to remember the past correctly. Harrow’s experience as a passive observer in her own story and her tenuous grasp on reality is a deeply personal reflection of the author’s own experiences with mental health. Muir writes in her Acknowledgements, “Harrowhark Nonagesimus did not have anyone to put soluble banana-flavoured antipsychotics under her tongue for her condition.