What is the plot and the significance of the main characters in the book The Alienist by Caleb Carr?

Caleb Carr's 1994 crime novel The Alienist follows John Moore, a New York Times crime reporter, as he and his friend Laszlo Kreizler (a famed psychiatrist) recount the grisly 1896 murder spree of a serial killer in Manhattan and their attempts to uncover his identity. 

Their journey begins on March 3, 1986, when Kreizler has Moore brought to the horrific crime scene, where a 13-year-old boy named Georgio "Gloria" Santorelli has been disfigured and killed: his eyes gouged out, right hand cut off, buttocks sheared off, throat slit, cut across his body, and genitals cut off and stuffed into his mouth. Santorelli was a sex worker who dressed as a girl in order to pick up johns. Despite this kind of case normally being ignored, Moore and Kreisler decide to investigate, and Kreizler realizes that there seems to be a connection to another murder case in which two children had had their eyes gouged out. 

Police Commissioner Theodore Roosevelt (yes, that Theodore Roosevelt... the future president!) understands that this is actually the third murder in this pattern, but to avoid controversy due to Kriezler's reputation, he decides to stay away from the case and allow Kriezler to work on it behind the scenes.

The pair are joined by detective brothers Lucius and Marcus Isaacson, as well as one of the first female employees of the NYPD, Sara Howard. Together, the team develops a profile of the serial killer in hopes of predicting his next attempted murder. They contact various mental institutions to see if anyone matching this profile had once been cared for in such an establishment.

After visiting Washington, DC, the team discovers another similar murder--this time, one that has taken place in rural New York. A visit to the site of this murder and a talk with the surviving brother of the killer reveals to them the killer's name: John Beecham (born Japheth Dury). The team discovers that Japheth had been unwanted by his parents and sexually abused by George Beecham, which was the cause of his vengeful behavior. Ultimately, John/Japheth is killed by a gang of thugs before Kreizler can interview him, and although he manages to conduct an autopsy of the body, only the knowledge of the killer's childhood of abuse can offer insight into his deranged actions.