MacSwain is an ex-cop, who now gambles away his money. He's at the center of a cover-up involving Noonan's brother, Tim. It was believed that Tim had committed suicide, but in fact he had been murdered. MacSwain's wife had fallen for Tim, but Tim had dumped her for Myrtle Jennison. But Myrtle was now Max's girlfriend. Yeah, love is complicated.
In a misguided attempt to defend his wife, MacSwain kills Tim but before Tim dies he whispers the word "Max" into Myrtle's ear. Myrtle assumes that Tim meant Max Thaler, so she bribes MacSwain to keep things quiet because she doesn't want Max to go to jail. MacSwain gets off the hook because he's the only one who realizes that Tim was in fact saying the first syllable of his name, "MacS." The Op finally pieces together this back story and informs Noonan that MacSwain is the one who murdered Tim.
Dan is a "lunger" (that means a man suffering from tuberculosis) and Dinah lets Dan live in her house and appears to be supporting him. He's in love with Dinah, but it's unclear what Dinah's feelings are for Dan. She's frequently shown slapping Dan around or making him run errands for her.
At one point in the novel, Dinah gives the Op information on Whisper, and when Whisper finds out, he sends his men to rough up Dan and Dinah. Dan suffers a mild concussion and winds up in the hospital. He later escapes, and it is implied that he kills Dawn, but the circumstances are never made clear. We also find out through a secondary source that Whisper has killed Dan, so we can add him to the body count.
Mickey belongs to the same San Francisco Detective Agency as the Continental Op. The Op sends for Mickey to provide additional assistance in cleaning up Personville. Mickey performs various jobs like tailing Pete the Finn. He is also the first person that the Op calls up after he finds Dinah murdered. Mickey suspects that the Op is the murderer, but he gives the Op the benefit of the doubt and doesn't turn him into the police.
Dick is a San Francisco detective who the Op requests to come assist him in Personville. The Op asks Dick to do various tasks like tailing Lew Yard and staking out the rented apartment of Helen Albury.
Albury is a bank cashier at the First National Bank. He provides the Op with the information that the $5,000 check signed by Donald Willsson was deposited by Dinah Brand. Albury later meets with the Op at his hotel and seems eager to give the Op any information he wants on Dinah, with whom he had had a brief fling.
Several chapters later, the Op shows up at the bank and reveals that he knows Albury murdered Donald. The Op explains that Albury was still in love with Dinah and thought that he was losing her to Donald. In a fit of jealousy, Albury followed Donald to Dinah's house and shoots him in the street. The murder weapon was a .32 caliber gun, the kind typically used in banks. Albury confesses to the crime and is sent to prison.
Lewis is Donald Willsson's secretary. She is the one who directs the Op's attention to the unaddressed $5,000 check that Willsson wrote out the day he was killed.
When the Op meets Myrtle, she's in the hospital, dying from an unknown disease. The Op traces her whereabouts through Dinah, who tells him that Myrtle knows Whisper had killed Tim Noonan. The Op asks Myrtle to sign a paper stating that she had bribed MacSwain to keep quiet about Tim's murder and that Tim had revealed the killer to be Max. The Op then uses this signed statement to entrap MacSwain into confessing that he was the actual murderer of Tim Noonan.
Bush is a boxer who has orders to take a dive during the fighting match because there's money riding on him to lose. But the Op gets a hold of him with information acquired through MacSwain. Apparently Bush's real name is Al Kennedy and he is wanted by the police for two counts of manslaughter. The Op makes a deal with Bush that if he doesn't flop during the game, then the Op won't tell the police. Bush decides not to flop, but at the end of the match, a knife shoots through the air and stabs Bush in the neck. He dies instantly.
Cooper is Bush's opponent in the fighting ring. He was supposed to win the fight, but Bush knocks him out.
Bill Quint supplies the Op with back story of how Elihu came to power in Personville. During World War I, Bill had worked for the I.W.W. (Industrial Workers of the World), and was sent to Personville to convince the Mining Corporation against a labor strike. Bill provides the Op with the names of the four men who hold most of the power in Personville: Pete, Yard, Whisper, and Noonan.
Dawn is a lawyer who convinces Helen to rent an apartment across the street from Dinah in order to steal love letters written to Dinah from Elihu Willsson. Dawn wants to use these letters to blackmail Willsson for money. He meets with the Op to tell him that he'll need legal representation to defend himself against the charge of murdering Dinah. But before Dawn can make his next move, he is murdered. The police think that the Op killed Dawn, but the Op is convinced that Dan did it. The details surrounding Dawn's death remain hazy, and the last bit of information the Op figures out is that it was Elihu himself who hired Dawn to recover the letters from Dinah.
Helen is Robert's sister, who believes that her brother has been wrongly accused of murder. She blames Dinah, so she rents an apartment across the street from Dinah's house. Helen follows the advice of the lawyer Charles Proctor Dawn, who devises a plan to steal a series of love letters from Dinah that were written by Elihu Willsson.
Murray is an old friend of Whisper Thaler. On the night of Tim Noonan's death, Murray lied to the police and corroborated Whisper's alibi that he was with the boys at the bar. However, by the time the Op steps into the picture, Murray and Whisper have had a falling out, so when Noonan and the Op question Murray again about Whisper's alibi, Murray says that Whisper had asked him to lie. This means that Whisper is the leading suspect in the murder of Tim Noonan.
Tim is the brother of Noonan the Chief of Police. His death had been written up as a suicide, but he was actually murdered by MacSwain. Before he died, he whispered the word "Max" into Myrtle's ear. She interprets this to mean that Max Thaler killed him, but in fact Tim was trying to say that "MacS", as in MacSwain, was the murderer.
Jerry is part of Whisper's crew. He is killed during a bank robbery to make it look like Whisper was behind it.
Tony belongs to Thaler's crew and is the man who kills Ike Bush for not diving during the boxing match.
Blackie belongs to Lew Yard's crew. We know nothing about him except that he is killed during one of the many gunfights.
Put Collings also belongs to Lew Yard's crew. We don't know anything about him either (notice a pattern?). He was killed along with his brother during a shootout.
Another unknown member of Lew Yard's crew. We're told that he used to be a guerilla, and he was killed in the same shootout as the Johnson brothers.