The Chief of Police in Personville is as corrupt as they come. He's on good terms with Lew Yard, and on bad terms with Whisper Thaler. In fact, Noonan seems to make it his personal goal in life to pin whatever crime he can on Whisper, regardless of whether the evidence points to him or not.
As one of the more unlikeable villains of the story, Noonan is usually happiest when he's emptying rounds of ammunition into empty warehouses in an effort to arrest Whisper. For the majority of the novel, Noonan functions as a pretty two-dimensional character without much psychological depth. Basically, he's bad news.
That said, there are two significant moments in the novel when Hammett allows us to sympathize more directly with Noonan. The first goes down when we discover that Noonan's brother Tim had been killed. Tim's death appeared to be a suicide, but the Op uncovers that Tim had actually been murdered. Although Hammett doesn't delve into the psychological effects of Tim's death on Noonan, we do get a sense that the reason Noonan is so intent on chasing after Whisper has something to do with Tim. The second scene in which Noonan becomes more than just a mere villain figure occurs when he confesses to the Op that he's sick of all the killing: "Everybody's killing everyone. When's it going to end?" (18.30). These aren't words that we would necessarily expect to be coming out of Noonan's mouth, but Hammett shows Noonan in a rare moment of human vulnerability.
Bonus: Given how many deaths take place in Red Harvest, it's worth noting that many of these murders take place "off stage," including Noonan's death.