Immunis Anagosa “Ana” Dolabra is the deuteragonist of the series. She is Din’s superior officer and a highly respected investigator among the Iudex. Ana is presented as being preternaturally brilliant, her intellect enough to triumph even against Sunus Pyktis, the novel’s antagonist, who has been supernaturally augmented to predict patterns, a skill he uses to concoct elaborate crimes. Along with Ana’s brilliance comes an idiosyncratic personality and a caustic attitude. She unreservedly insults those whom she dislikes, only occasionally bowing to the military hierarchy of Khanum.

Ana has many habits that others view as strange, including wearing a blindfold to limit her visual input. Her high sensory acuity helps her solve crimes but which also leaves her easily overwhelmed. Ana delights in testing her senses by eating foods that Din often finds horrifying; in A Drop of Corruption, she eats hundreds of oysters in a single sitting to determine if she can taste the differences between them, and she later asks for a feast of “many exotic classes of flesh” to help her summon her investigative prowess (288). At the end of the novel, Din learns that some of this strange behavior may be attributable to Ana’s likely status as a descendant of the ancient (and extinct) race of Khanum, the first people of the Empire. Ana hints that she is a “remade” member of this group, but that she has been magically prevented from saying more by an extreme augmentation that makes it physically impossible to reveal a certain secret. As Ana cannot explain matters herself due to this limitation, any differences between her and the original people of Khanum remain a mystery at the end of this installment.

Despite her many oddities, Ana is presented as the immovable moral center of the novel. No matter how complex the mystery she faces or the political situation in which she finds herself, Ana stands by her sense of right and wrong. Her incorruptibility helps shift Din’s own attitude toward the Iudex, as he ultimately recognizes that the investigative organization does the essential work of reining in corruption in the Empire. A mentor figure to Din, she is a mostly static character: the fixed point against which Din is able to measure his growth.

While Ana is the deuteragonist of the series, Signum Tira Malo is the deuteragonist of A Drop of Corruption, developing a friendship with Din as they investigate the murders in Yarrowdale together. Malo is a warden of the Apothetikals with highly augmented senses, meaning she can hear, see, and smell things that Din and Ana cannot. Malo uses these skills throughout the investigation.

As a person of Yarrow descent who works as a member of the Imperial military, Malo understands both sides of the political negotiations between the Yarrow king and the Treasury department. Malo reveals to Din that she was born a naukari, a member of a group of “ancestral servants.” She explains that while the naukari are functionally enslaved, the legal nuances of Yarrow distinguish the naukari’s position from illegal enslavement because naukari cannot be sold or traded; instead, their labor is bound to the land they work, not any particular noble. This personal history heightens Malo’s determination to see the naukari freed, and, when Yarrow’s court collapses at the end of the novel, she stealthily helps as many naukari as possible to freedom.

Malo is prickly and often snipes at Din; in this regard, she is similar to Ana, though Malo and Din form a more equitable friendship than the mentorship Din shares with Ana. This may be due to Din and Malo’s shared military rank. Malo provides insight into the political situation in Yarrow as understood by the common folk of the country, rather than its king. Malo worries both about her own future and about the future of her countrymen when the annexation to the Empire is cancelled, as Khanum has access to a great deal more wealth and technology than does Yarrow. She thus represents the danger of kings to the people of a kingdom, something that becomes increasingly visible to Din as he befriends her.

At the end of the novel, Din suggests to Ana that she help Malo find work in the Iudex, as he feels that Malo would excel at such work. The text implies that Malo may be a recurring character in the series via a vague reference to Din and Malo encountering one another again.

Immunis Sunus Pyktis is the main antagonist of A Drop of Corruption. He appears most frequently in the novel disguised as Prince Carmak of Yarrow, who is later revealed to have been Pyktis’s twin, whom he murdered in order to steal his place as heir to the Yarrow throne. Pyktis is one of the Yarrow king’s many children. He was sent to the Empire as a child to work as a secret agent on behalf of his home country: With a legitimate history growing up in Khanum, he would be eligible to apply to the Apothetikals. He was tasked to destroy the Shroud, something that would increase Yarrow’s negotiating capacity with the Empire. After failing in this task, Pyktis sought aid from his father, the king, only to learn that the elderly man’s memory problems had led him to forget his son entirely.

Disillusioned with monarchy, Pyktis sought to use his skills as an augur, given to him during his work on the Shroud, to kill his brother and father, take the Yarrow crown, and then destroy the living titan’s marrow, which the Apoths of the Shroud had learned to transport safely. To this end, he sowed chaos across Yarrowdale so that the marrow would be transported in haste. Pyktis rationalizes his crimes by citing the corruption of the previous king before him, framing himself as stuck between being the exploiter or the exploited, an explanation that Ana scorns. Pyktis dies at the end of the novel when Thale Pavitar, a courtier loyal to the Yarrow crown, kills Pyktis for his crimes against his family, a serious taboo in Yarrow culture.