John Divney is mentioned on the first page of the novel as the narrator’s accomplice in the murder of Phillip Matthews. He is described as “a strong civil man,” although “lazy and idle-minded” (7). Divney is secretive and callous after the murder. He refuses to tell the narrator where he has hidden the money box; when they return to the farm, he quotes what he said to distract Mathers before hitting him, then lets “out a bellow of laughter which seemed to loosen up his whole body” (18). He is characterized by the narrator as dishonest; the narrator decides that Divney is likely to take his share of Mathers’s money, and that he cannot let the other man out of his sight.

Near the conclusion of the novel, Divney reveals that he murdered the narrator as well by placing a bomb in the floorboards of Mathers’s house and telling the narrator the money box was there. He experiences extreme shock and guilt when he sees the narrator, and dies of fright, joining the narrator in hell.

Mathers is a wealthy neighbor who “[h]ad spent a long life of fifty years in the cattle trade” (15). He is characterized as a rich recluse by Divney and the narrator before his murder. When the narrator meets Mathers after he has been murdered, he is surprised to like his victim. Mathers says “no” constantly, and espouses a philosophy that replying in the negative is preferable to replying in the positive, because “yes” is more likely to lead to sin. Rather than a developed character, Mathers functions primarily as a representation of the narrator’s guilt.

Martin Finnucane is a robber the narrator meets on the road between Mathers’s house and the police barracks. Finnucane is “tricky and smoked a tricky pipe and his hand was quavery. His eyes were tricky also probably from watching policemen” (45). 

The narrator is initially fearful of Finnucane, and Joe views him as “a very slippery-looking customer” (45). He threatens the narrator’s life casually, and does not value his own, suggesting that life is useless because you cannot smoke or eat it and because it is “a certain death trap” (47). However, the narrator begins to like and feel an affinity for Finnucane when he realizes the latter also has a wooden leg, rendering Finnucane a double for the narrator both physically and in terms of his criminal intentions.

Finnucane is the “captain of the one-legged men in this country” (49) and offers to save the narrator if he ever needs it. When the narrator is about to be hanged, he calls for Finnucane, who attempts to come rescue him. Finnucane functions as a symbol of hope as well as an alternate self for the narrator.

Joe is the narrator’s soul, who begins speaking to him while he is in Old Mathers’s house. Joe is impressed with Mathers’s philosophy. Joe is imaginative and wryly critical of the narrator. When the narrator does not know his name and is imagining potential options, Joe provides elaborate summaries of identities like “Signor Beniamino Bari […] the eminent tenor” (43) and “Dr. Solway Garr” (44), telling the narrator that they are “only a hint of the pretensions and vanity that you inwardly permit yourself” (44). As well as being critical of the narrator, Joe is sensitive. When the narrator thinks about Joe’s imagined body as being scaly, Joe takes extreme offense and threatens to leave.

The narrator describes the sergeant as an “enormous policeman” with a large body, and straw-colored hair. The narrator notes that although each part of him looked normal if taken alone, but “they all seemed to create together […] a very disquieting impression of unnaturalness, amounting almost to what was horrible and monstrous” (56). He has a red face and “silent red mustache” (57). Despite his appearance, the narrator thinks he looks to have a “good nature, politeness, and infinite patience” (57). He initially appears to be needlessly obsessed with bicycles, returning all conversations to related topics. He is a comedic character, and his words and actions are characterized by absurdity.

However, the narrator gradually begins to experience increasing complexities in Pluck’s character. He appears more sinister when he reveals that he is the one who stole Gilhaney’s bicycle. However, this turns out to be in part a gesture of protection for the community members for whom he is responsible. He steals the bicycle to slow the progress of Gilhaney turning into a bicycle and the bicycle turning into a human.

MacCruiskeen is “heavy-fleshed,” with “black curly hair,” appearing more intelligent than the Sergeant. He speaks with a high, delicate voice. MacCruiskeen is an inventor and gifted at craftsmanship, having built the series of beautiful, increasingly miniscule boxes, spear with the invisible point, and the small musical instrument. He is later described as malicious, enjoying the thought of attacking the band of one-legged men with the “unbeholdable” paint color on his bicycle.

Mr. Gilhaney is the other civilian who spends time at the police barracks during the novel. He is there about a stolen bicycle, which it transpires the Sergeant himself has taken. He is important primarily because he is a clear representation of a human who has become like his bicycle. The Sergeant describes him as being nearly 60 years old, and 48% bicycle. He needs to continue moving forward or lean on things to avoid toppling over. He once forgets, and “made the mistake of stopping dead completely instead of keeping on the move to preserve his perpendicular balance” (116), falling into MacCruiskeen. Like the Sergeant, Gilhaney talks almost exclusively about bicycles.

The titular “third policeman,” Policeman Fox is characterized initially by Sergeant Pluck’s description of him as eccentric and seldom seen. Pluck says that Fox is “as crazy as tuppence-half-penny and as cranky as thruppence” (79). Pluck also suggests that this has been the case since an extended private conversation with MacCruiskeen, more details of which are not provided. He sleeps in the police barracks near the narrator one night, though the latter does not realize he was there. Whereas the Sergeant and MacCruiskeen sleep down in eternity to prevent aging, Fox sleeps in the barracks because “he wants to get rid of as much [life] as possible, undertone and overtime, as quickly as he can so that he can die as soon as possible” (158). 

Policeman Fox does not appear until late in the novel. When he does, the narrator sees old Mathers’s head atop the policeman’s body, so his identity and hallucinatory nature is unclear. He is described as large and fat, with a red face.