|“||I am like a Boy Scout, sir. Always prepared.||”|
— Kate in "X"
Katherine Callahan is a seasoned FBI agent who joins the BAU during the premiere episode of Season Ten after Alex Blake left the BAU. She then left the team in the season finale due to her pregnancy. Her position was taken over by Dr. Tara Lewis in Season Eleven.
Very little is currently known about Kate's early years. She has a background in criminal psychology and worked as an agent for the FBI's Sex Crimes Unit for eight years, with her specialty being undercover operations. Her work has allowed her to establish a prior friendship with the BAU team. It was also mentioned that she once worked with SSA Andi Swann and her Domestic Trafficking Task Force.
In her personal life, she is in a committed, stable marriage with a man named Chris. She went through a big loss during the 9/11 terrorist attacks, as her sister Liz and brother-in-law Joe were among the thousands of victims. The two of them were working at the Pentagon when American Airlines Flight 77 was flown into it by Al-Qaeda terrorists. Since then, she has become the guardian for their infant daughter, Meg, who she has since raised for the past thirteen years. All of these happenings have shaped her patriotic attitude.
According to Erica Messer, Kate is "smart, charming, and wise for her years". She also holds a passion in making the world a safer place and is a "nurturing figure" in her personal and professional lives. Kate also is willing to "go to great lengths to get her job done". Because it takes her months to capture a criminal, whereas the BAU captures their unsubs weekly, she finds herself having to adapt to that schedule. She is extremely skilled at manipulating criminals, who usually fall for her beauty and giving her a leeway in her cases.
Kate joins the BAU in X after Hotch sees her record and is impressed by its credentials. She was the tenth candidate to take over Blake's position. She then immediately joins the rest of the BAU on the search of a serial killer called "The Mad Butcher of Bakersfield", who has already claimed three lives within a month-long span. During the investigation, Kate befriends Garcia over their shared knowledge of cute photos. She spends most of her time accompanying Rossi, first to the crime scene and then when they approach a suspect named Frank Cowles, who purchased an unusual amount of preservatives, which they suspect is for the preservation of limbs taken from the victims. She and Rossi bait Cowles into taking the limbs with him out of the house, and once he drives onto an abandoned trail, she leads the arrest on him. After deducing that Cowles was obsessed with limbs but doesn't have the capacity to kill people, Kate accompanies the rest of the BAU to the secondary residence of the real unsub, identified as Steven Parkett. There, they find a would-be victim who escaped, and the team splits up to find Parkett. Eventually, Kate is attacked by Parkett, who tries to kill her with a machete, but is shot and killed by Hotch. At the end of the episode, Kate reunites with Meg and tells her that she loves her.
In A Thousand Suns, after a passenger jet suspiciously crashes in Colorado, she tells Reid about the deaths of her sister and brother-in-law on 9/11. During the entire episode, she is shown visibly affected by the case, which bears eerie similarities to 9/11. By the case's conclusion, Kate watches as Hotch shoots the unsub dead before he could commit another 9/11-style attack. At the end of the episode, she is seen standing with Reid and JJ on a memorial to the unsub's victims, overlooking the scene of the plane crash. She receives a call from Meg, to whom she tells her love too.
In Scream, Kate is shocked to find out that Meg and a friend of hers, Markayla Davis, are planning on going out to the shopping mall with a pair of male classmates from their biology class. To her dismay, Chris forgets about the contingency plan they set up for the occasion that Meg would go out on her first date. By the case's conclusion, Kate shoots and kills the unsub as he is in the process of killing JJ and a would-be victim. When she returns home, she finds out from Chris that he secretly followed Meg and Markayla, and that they were stood up for the date. Little did Chris know a man was stalking Meg and Markayla, and had been intending to abduct them.
- Her sidearm is a Glock 19.
- She sings karaoke. It is unknown how good she is at it. (X)
- She was the tenth candidate applying for the vacancy left by Blake. (X)
- She likes cute photos of animals. (X)
- She reveals herself to be very clumsy, remarking that her mother once told her as a girl that if clumsiness was a virtue, she'd be St. Katherine. (If the Shoe Fits)
- She hates prisons. (Boxed In, Lockdown)
- She is able to know how many football fans live in a city through Google. (The Boys of Sudworth Place)
- She once took an FBI class taught by Gideon, although he was only present for two lectures before he left to rejoin the BAU and investigate the Seattle Strangler and Footpath Killer cases. (Nelson's Sparrow)
- She owns three guns. It is unknown if at least one of them is her FBI-issued sidearm. (Scream)
- Her earliest childhood fear was a girl named Melissa Gordoner, who was presumably the neighborhood bully. (Mr. Scratch)
- The name Melissa Gordoner could be a reference to the character Jennifer Love Hewitt plays in Ghost Whisperer, Melinda Gordon.
- She takes a year off to be with her baby, like she did when she first got Meg. (The Hunt)
- In reality, Kate was written out of the show due to her actress Jennifer Love Hewitt's real-life pregnancy, and Hewitt too wanted to focus on her baby.
- She has currently appeared in a total of 23 episodes, The Hunt being her last.