(Adam) Rob Ryan
In 1984, Rob's two best friends went missing in the woods. They were never seen again but Rob, who was with them at the time, was found clinging to a tree with blood in his shoes. He creates a new persona for himself so that he is not forever going to be "the boy in the woods" although he cannot remember anything that happened at all that night anyway and would not be able to talk much about it if anyone asked him. The persona has made him a gifted liar, although he does not lie from a place of malicious intent, but rather to prevent people from knowing the childhood that is his truth.
It's unclear whether of not Rob subconsciously hopes that working as a murder detective might trigger a memory about the night in question. He is a good detective, though, and able to think in clear steps which enables him to come to some concrete conclusions and ultimately realize how Katy came to die.
Romantically, Rob is a disaster. He likes damsels in distress and when they turn out to be in less distress than he first thought, he finds that he is no longer attracted to them. Cassie is a prime example of this; when he first meets her she appears helpless, and he is immediately attracted to her. When he realizes how tough and capable she is, the attraction wanes. However, he is also a man who doesn't know what he has until it's gone; after sleeping with Cassie, he acts poorly, and makes their partnership awkward, because he doesn't want to have a romantic relationship with her. When she gets engaged to someone else, later on, he begs her to give him another chance because he realizes that he does want to be with her after all.
Cassie Maddox
Cassie is an incredible cop, far more impressive than Rob, although he feels that he is the stronger member of the partnership. She has survived being stabbed whilst working undercover and is one of the youngest ever recruits to Murder Squad. She takes this all in stride and is professional and confident without becoming a cocky braggart.
Cassie has endured several life experiences that make her the perfect detective to investigate Katy's case. As a child she was almost molested by the janitor at school and so she is able to imagine how Katy must have been feeling when she realizes that something terrible is about to happen to her. She was also once stalked by a psychopath and is therefore able to profile the kind of person that they are looking for in their investigation. She is almost wasted as a murder squad detective; her niche seems to be a behavioral science unit.
Cassie does make an error in that she takes Rob's word for it that he has investigated Rosalind properly; informed by him that she is eighteen years old, Cassie records Rosalind's confession only to find out that it cannot be used because she is a minor and did not have an appropriate adult with her when she made the statement.
Rosalind Devlin
As murder victims' sisters go, Rosalind at first seems like an appropriately sympathetic character, but we come to realize that she is a psychopath with no empathy whatsoever. She is also a narcissist which makes her an easy liar, and conscience-free. If there was a perfect resume for a murderer, Rosalind would definitely have it.
Rosalind is jealous of Katy. She wants her dead, because Katy has a mind of her own and will not allow herself to be bossed around. She tries to poison her over time but it seems to slow. Rosalind does not actually kill Katy herself, but uses her boyfriend, Damien, to do so; in a way, she is the killer, and Damien is her weapon. She plays upon his need to protect her and tells him that she and her sisters are all sexually abused by their father, but that Katy enjoys it and encourages him to physically abuse the other two whilst she watches. This manipulates him into doing what he thinks is the wrong thing for the right reasons.
Even at seventeen, Regina is well aware that she can wrap men around her little finger and does so, manipulating Rob and convincing him that she is eighteen years old,not seventeen, which makes her confession inadmissible. It is extremely likely that she will kill again.
Damien Donnelly
Rosalind's boyfriend Damien is an archaeologist and frankly appears to be a bit of a nerd. Rob interviews him as routine, and dismisses him because he believes he is too dorky to be a murderer. He is not featured in much of the narrative until the end when he confesses to Katy's murder, which he committed in order to protect Rosalind from the rage of their abusive father, which Katy encourages (according to Rosalind.)
Damien was not the brains behind the murder but he does seem to have a pretty good idea of how forensics work. He "finds" Katy's body and admits to touching it, which effectively explains any evidence that relates to him on the body. He did not come up with the plan for the murder but he definitely came up with the plan that would enable him to avoid detection.
Sam O'Neill
Sam is a detective who is called in to assist in the investigation. Although it is Rob who calls him in, the two men are like chalk and cheese. Rob is extremely intelligent and erudite and has a charm that Rob does not. He is related to a politician who was once influential but was subsequently revealed to be crooked.
Katy Devlin
Katy is the murder victim, and is found by Danny at the site of the archaeological dig. She has a twin sister, Jessica, who has developmental difficulties. The only real description of Katy that we have comes from Rosalind, who paints her as a kind of willful sadist who gangs up with their father, with whom she is enjoying a sexual relationship. This is almost certainly untrue; it is likely that Katy refused to allow Rosalind to manipulate her in the same way that she manipulates everyone else, and this is what Rosalind decided to kill her for.
