Shawn Spencer Born in 1977, Shawn hails from a family of police officers. His parents separated in 1992, and later divorced; his mother, Madeleine (a psychologist), eventually returned and explained that she left to pursue a job opportunity. His father, Henry Spencer, fully expected him to carry the torch, and schooled Shawn in detective work from as early as age 7, aiding the development of his extraordinary powers of observation and deduction. At the age of 15, he achieved a perfect score of 100 on the detective's exam. This is drawn loosely from the childhood of series creator Steve Franks, who was also born into a family of police officers and whose father "was training [Franks] in his own way to follow his footsteps." Franks' father would ask him how many people were wearing hats in a restaurant, a tactic employed in the show's pilot episode by Shawn's father. Shawn's relationship with his father is complicated and at times highly strained. Though often it may seem that Henry is intentionally aggravating or imposing his will on his grown son, it is usually revealed that his motives are genuine. For example, tipping off the meter maid to ticket Shawn's motorcycle which is parked too close to a hydrant outside his apartment. The impounding as a result was Henry's ultimate goal, as Shawn had got into an accident some time ago. Despite his obvious talent, Shawn has no interest in entering the police force, and after graduating from Leland Bosseigh High School in Santa Barbara, California, in 1995, takes up a string of random jobs selected strictly for his own enjoyment. In his spare time, he entertains himself by calling in tips to the police hotline based on observations he has made on crime scenes shown in television news footage. After one such tip, the Santa Barbara police become suspicious of Shawn, whose information is so good that they believe it could only have come from an inside source. Desperate to avoid jail time, Shawn lies, explaining that he obtained the information in a psychic vision. Although initially skeptical, the department is impressed when he uses his "psychic" deductions to deduce that McNab was getting married, the secretary was haunted by her dead grandmother, Lassitter was in a relationship with his partner, and solve a vandalism case. So they hire Shawn as a consultant. Enlisting the assistance of his reluctant best friend, Burton Guster, Shawn opens his own psychic detective agency, "Psych". It has been recently revealed he inherited his eidetic memory from his mother.

Juliet O'Hara first appears in the show's second episode, "Spellingg Bee", as a recently-transferred junior detective from Miami Beach, replacing Lucinda Barry as Head Detective Carlton Lassiter's partner. She was raised in a family of brothers and is shown to be close with both her parents and her siblings. Although she is initially frustrated by her coworkers not taking her seriously enough due to her relatively young age, as the series progresses she is given more responsibility, being left in charge in the absence of Lassiter and Interim Chief Karen Vick and getting her first undercover assignment. She has grown more assertive with her partner, and actors Maggie Lawson and Timothy Omundson (who plays Lassiter) both suggest that a mutual respect has developed between their characters.

In the episode "Six Feet Under the Sea", which originally aired on January 9, 2009, Juliet is talking to a former aquarium worker who asks her about Shawn because she (the aquarium worker) might like to go on a date with him. Juliet's reaction implies that she has feelings for Shawn. At the end of the episode Shawn asks out the aquarium worker, but she turns him down because she likes Juliet and doesn't want to ruin it for them. She wishes Shawn "good luck," leaving him confused. At the end of episode 13 "Any Given Friday Night at 10PM, 9PM Central", Juliet smiles as she see, that Shawn plays real football and not only practice it. In the third season finale, "An Evening With Mr. Yang," Juliet finally asks out Shawn on a date, and owns up to the fact that she has been sending mixed signals. Unfortunately, Shawn tells her the timing is awful, as he's on a date with his old high school flame, Abigail. When Shawn explains the situation, Juliet understands. She kisses him on the cheek and leaves. Shawn is obviously affected by the kiss but rejoins Abigail and seems quite prepared to proceed with that relationship. However, Gus acts as a chaperone of sorts for Shawn and Abigail's date, which Shawn attributes to Gus' overprotectiveness of the company car, but a look exchanged between Shawn and Gus suggests that Shawn may have consented to Gus' presence due to his feelings for Juliet.

Carlton Lassiter is a ten-year veteran of the Santa Barbara Police Department, and the youngest Head Detective ever named to the force. According to Lassiter's blog, which is written by actor Timothy Omundson, he was a close friend of the former Santa Barbara police chief, who led him to enroll in the police academy during a "dangerous point" in Lassiter's youth. Upon his first appearance in the series pilot, Lassiter is said to be five months into a trial separation from his wife, Victoria, although he later, when drunk, confesses that they have in fact been separated for two years. Part of their marriage troubles appears to stem from a disagreement over the matter of children. Lassiter says his wife believes that he does not want children, although this is not the case. Omundson says that, despite Lassiter's long estrangement from his wife, it would likely be very hard for him admit that the marriage is over because "that means admitting failure, and Lassiter will never admit to failure. During the pilot, Lassiter is involved in a secret romance with his partner, Detective Lucinda Barry; however, soon after Shawn Spencer correctly determines the nature of their relationship, Barry is transferred and the affair appears to have ended. She is replaced by Junior Detective Juliet O'Hara. Eventually, Lassiter's wife asks him to dinner, something he thinks points to a reconciliation for them, but in truth she simply wants to give him divorce papers. Saddened but resigned, he signs them, telling her he's going to let go of the past and think of the future without her. She tells him she will never stop loving him, even though they are divorced. It is revealed in Lassiter's blog that he has a sister, a missing brother who may or may not be in South America, a brother in law named Raul, a nephew named Peter, and an overbearing mother and grandmother.

Henry Spencer is a Canadian computer programmer and space enthusiast. He wrote 'regex', a widely-used software library for regular expressions, and co-wrote C News, a Usenet server program. He also authored The Ten Commandments for C Programmers. He is coauthor, with David Lawrence, of the book Managing Usenet . While working at the University of Toronto he ran the first active Usenet site outside the US, starting in 1981. His records from that period were eventually acquired by Google to provide an archive of Usenet in the 1980s.

The first international Usenet site was run in Ottawa, in 1981; however, it is generally not remembered, as it served merely as a read-only medium. Later in 1981, Spencer acquired a Usenet feed from Duke University, and brought "utzoo" online; the earliest public archives of Usenet date from May 1981 as a result.

The small size of Usenet in its youthful days, and Spencer's early involvement, made him a well-recognised participant; among other things, this is commemorated in Vernor Vinge's 1992 novel A Fire Upon the Deep. The novel featured an interstellar communications medium remarkably similar to Usenet, down to the author including spurious message headers; one of the characters who appeared solely through postings to this was modelled on Spencer (and, slightly obliquely, named for him).

Karen Vick replaced Police Chief John Fenich upon his retirement about a month prior to Shawn's arrival at the SBPD. However, in Poker? I Barely Know Her, she references an event known as "The Secret Santa Debacle of 2005", suggesting that she has been involved in the Santa Barbara Police Department in some capacity at least since December of that year. Vick's relationship with Lassiter seems strained by the fact that she is "an outsider" who was chosen by "the powers that be in City Hall" to take the position over "someone from within our own ranks." Vick's decision to allow a self-proclaimed psychic so far into official police investigations has done little to assuage Lassiter's misgivings. Eventually, Lassiter's attitude towards both Shawn and Vick soothed.

On her way to non-lethal weapons training with Lassiter, Vick's water broke and Lassiter rushed her to a hospital where she gave birth to a daughter.

Since Nelson was pregnant during the filming of the pilot episode and gave birth shortly thereafter, for continuity she had to wear a fake pregnancy prothesis for several of the following episodes.

For Seasons 1 and 2, her position as Police Chief was described as 'Interim.' At the very end of Season 2, Vick announced that she would step down because her position as Chief was not solidified by the mayor, but she received a call from the mayor soon after, effectively giving her the position. This was mainly because her replacement had been exposed as an adulterer by Shawn.