Dr. Kreizler is a psychiatrist or the “alienist” who also works with the murder case.
He is of great help to the investigation team.
He shows the team many of his psychology books & articles so that they can all try to understand what goes on the in mind of the killer.
Dr. Kreizler does numerous evaluations of murderers & analyzes the info to see if it helps with the case of Gregorio.
Later on, a fellow psychologist agreed that a female in the killer’s childhood most likely contributed to the killer’s psychological state.
Kreizler realizes that he experienced the “psychologist’s fallacy” and shows his humility by apologizing to Sara (the investigator that proposed the belief in question).
Later on in the novel, we find out that the reason why Dr. Kreizler refused to believe that a female played an active role in the killer’s childhood was because Kreizler himself was physically abused by his father as a child & his mother did nothing about it .
He shows a bit of hardheadedness in his character when he refuses to accept another investigator’s belief that a female played an active role in the killer’s childhood.
Sara plays an important role in the investigation team, she is the liason.
There is a point in the novel in which Moore suspects that Dr. Kreizler and Sara are romantically involved.
This idea is never confirmed though.
Sara contributes greatly to the investigation team.
Sara is the one who brought up the important idea that a female played an active role in the killer’s childhood.
Sara also shows that she is considerate and respects privacy because she and Moore did not reveal to anyone else- the personal information about Dr. Kreizler and how he was physically abused by his father as a child.
Sara also shows bravery in her character when she jumps in and shoots the head mob leader to save Moore and Dr, Kreizler.