Jennifer Jareau

Jennifer "JJ" Jareau is a fictional character from the CBS crime drama "Criminal Minds", portrayed by A. J. Cook.
Character background.
During various episodes, Jareau has described growing up in a small town near Pittsburgh, Pennsylvania. When she was eleven, her seventeen-year-old sister Rosalyn died by suicide. Rosalyn's death was a particularly painful topic for her and she initially chose not to tell her young son about Rosalyn, until her mother Sandy reminded her that avoidance was not the solution to dealing with the pain. A high school athlete and scholar, Jareau graduated valedictorian from East Allegheny High School, where she was the captain of the varsity soccer team her senior year and earned an athletic scholarship to the University of Pittsburgh. She also attended Georgetown, presumably for graduate studies.
JJ decided to join the FBI after attending a book reading by BAU founder and then-Unit Chief Agent David Rossi. She once told Rossi that she had not known what to do with her life until she listened to his reading, which was held in the campus bookstore during her senior year in college. The next fall, she applied to the FBI Academy.
Storylines.
In the FBI's Behavioral Analysis Unit (BAU), JJ acted as the team's liaison with the media and local police agencies. Though talented and helpful, she was not actually a profiler, having once declined Unit Chief Aaron Hotchner's suggestion to take the necessary classes in behavioral analysis. She works mostly out of the confines of the police stations and field offices the team visits. However, she does accompany the team on raids, and is proficient with firearms. This is made particularly clear when JJ resolves a hostage situation inside the BAU by killing the hostage-taker, Jason Clark Battle, with a single shot (Battle is a serial perpetrator of hero homicide, who lured Technical Analyst Penelope Garcia onto a date, and then shot her, in an attempt to keep his crimes from being discovered). During season six, JJ is given a promotion to the Pentagon; however, she returns in season seven as a full-time profiler, having received the necessary training. In season 9 ("200"), she is then kidnapped, and it is revealed that her year as the DOD Liaison at the Pentagon was a cover story. It turns out she was on a task force in Afghanistan, in search for Bin Laden. The team saves her, and in season 10, it is revealed she has PTSD, and hasn't been dealing with her ordeal being tortured when she was kidnapped, as well as two main events that happened when she was in Afghanistan, one of which includes her having a miscarriage, after a convoy was attacked. She tries to work through it with the help of Reid, who calls her on it when she disappears into the morgue one day for almost a half-hour after seeing herself on the table in place of the victim.
Promotion to Pentagon and brief return.
It is revealed in "JJ," that JJ had twice rejected a promotion to The Pentagon and hid the offers from her superiors. However, she is finally forced to transfer by "the brass." JJ's departure reflects that Cook was let go at the beginning of season six, for financial reasons relating to the premiere of the spin-off "." JJ's time at the Pentagon was revealed in depth in season nine.
Cook later returned for the departure of Paget Brewster (Agent Emily Prentiss). In the episode titled "Lauren," JJ receives a call and returns to help the BAU find Emily and capture Ian Doyle. The episode ends with Emily presumed dead after being stabbed with a broken chair leg in the stomach. It is later revealed that Emily is alive and in Paris with JJ's help, providing her passports from three different countries and a bank account in each "to keep her comfortable." In the season six finale, JJ returns in order to tell David Rossi that she is "coming back."
In "200," it was revealed that JJ's transfer was a backstop and that she was actually put on a task force in the Middle East with Mateo Cruz (who later became the section chief of the BAU following Strauss's death). During this time, she witnessed a mission she had pushed for fail and discovered one of the task force's informants dead. In February 2011, while overseas, she discovered that she was pregnant. However, in April 2011, following the events of "Lauren," during an attack on the task force, she was wounded and miscarried the baby. She didn't initially tell Will of the pregnancy, and it remained a secret between her and Cruz. In "The Forever People," when she starts showing signs of PTSD, Reid calls her on it, and he tries to help her and talk to her. She eventually admits she had a miscarriage due to the attack on her convoy, and asks him to keep it between them.
Return to the BAU.
The season seven premiere reveals that JJ has returned to the Behavioral Analysis Unit, having received the training to become a full-time profiler, instead of the Media Liaison, a position which has been split between Hotch and Garcia.
She is initially against summoning Prentiss "back from the dead," but Hotch insists on telling the team the truth. Also, she learns Reid is emotionally hurt by her deception, as she knew he cried for ten weeks after losing Prentiss and she never said a word, and Reid revealed he considered taking Dilaudid again.
In season nine, the events during her departure in 2010–11 returned when Mateo Cruz was promoted to Section Chief. In "The Road Home," while waiting to meet Cruz, she is suddenly kidnapped. In "200," it is revealed that she was kidnapped by a man named Tivon Askari, who was a traitor in the task force, in order to access files from her mission in the Middle East. She was held hostage alongside Cruz and was tortured into giving up the code. She discovered that Askari was not working alone when he mentioned her miscarriage, revealing the true enemy to be Michael Hastings, a man she worked with on the task force. He threatened to rape JJ in order to get Cruz to give up his access code. She is rescued by Hotch and Prentiss, and she proceeds to chase after Hastings. Prentiss accompanies her and they both take on Hastings, resulting in Hastings falling off a roof to his death. JJ is left traumatized by the situation and promises to stop the lies and give only the truth.
A year after her kidnapping and torture, she starts experiencing PTSD, something that Reid notices. When she is confronted by him, she asks him to provide a term to describe her situation, saying that it's more than PTSD. She breaks down and reveals the source of her PTSD to be Tivon Askari and reveals her miscarriage to Reid. She later takes an extremely large risk against an UnSub in a room with a gas leak, which could have resulted in the deaths of herself, the UnSub, and a hostage. Reid approaches her in the BAU to tell her that while he couldn't find a term to describe JJ's condition, he called Emily and managed to obtain Askari's file, revealing his background of his life on the streets of Tikrit, Iraq, and his criminal background. JJ unknowingly hallucinates Askari in the room with her. He taunts her and tells her that he wants to watch her kill herself. He also tells her that he will take her sleep, her smile, and her safety away, as well as change her so that her husband and son won't even recognise her. He explains that the memories will eventually take away everything she holds dear, but JJ snarls a harsh "No" to Askari before realizing it was all a hallucination.
Relationships and family.
JJ is married to Will LaMontagne. They have two sons Henry and Michael. As of season 13 she and Matt Simmons are the only members of the BAU who are married with children.
Prior to her courtship and marriage to Will, JJ displayed little interest in dating. She once went on a date with Spencer Reid to a Washington Redskins game, but little came of it romantically, although the two became very good friends. In the season 2 episode "Jones," JJ first meets Will LaMontagne a NOPD detective whom the team worked with while in New Orleans after a serial killer presumed dead following Hurricane Katrina resurfaces. It is revealed the following season, in the episode "In Heat," that he and JJ had been seeing each other for the past year but kept it a secret. JJ said she wanted to keep her personal life away from the prying eyes of her colleagues, which Will initially misunderstood as her being ashamed of their relationship. When she finally reveals their relationship by kissing him goodbye after the team solves a case in Miami, it is apparent that her colleagues already knew about them.
In "The Crossing," JJ told Will that she was pregnant with his child. He later asked her to marry him, and indicated that he was willing to give up his detective's shield in order to help take care of the baby and allow JJ to continue to do her job, though he was worried about her safety and the safety of their child. JJ later told Hotch she had found a temporary replacement for the duration of her maternity leave, and soon after gave birth to a baby boy named Henry. Will transfers to the MPDC, and the family live together in Washington. In later seasons, toddler Henry is portrayed by actress A. J. Cook's real-life son Mekhai Andersen. JJ and Will named Reid and Technical Analyst Penelope Garcia as Henry's godparents. When JJ returned from maternity leave, she and Will had exchanged rings with Henry's birthstone, but did not seem to be officially engaged, as she wore her ring on her right hand, not the left. In the season seven finale, JJ tearfully asked Will to propose to her again, and they married the following evening in a small ceremony in David Rossi's backyard.
In "Truth or Dare", the Season 14 finale, JJ confesses to Reid that she has loved him ever since they first met. Despite insisting that she only said it to distract a killer holding them hostage, Reid knows what she said is true but chooses not to dwell on it for the sake of their friendship. Season 15 opens up "Under the Skin" with JJ being shot and clinging to life. At the hospital Reid admits to a sleeping JJ that it does not matter if her feelings are true or not, he cannot imagine life without her. JJ wakes up and eventually tells Reid that what she said was true and he'll always be her first true love and hold a part of her heart. Despite the shared loved between the two, JJ and Reid keep this a secret and remain best friends.
Characterization.
JJ is very loyal to her family and friends, as well as very protective. She is also known to be a crack shot. This was demonstrated in many episodes such as "Nanny Dearest" by shooting a fleeing suspect in the head at a faraway distance. In "Revelations," she killed numerous flesh eating dogs in a dark barn. In "Penelope," she kills Garcia's attacker by shooting him directly between the eyes precisely as he turns around to find JJ behind him. Despite her petite frame, she is very athletic and has shown herself to be able to hold her own even if disarmed. In the season 7 finale, she manages to physically fight off a stronger and taller suspect who had tried to kill Will and took their son Henry hostage. She is best friends with Spencer (and is the only one who calls him "Spence", besides Penelope and Emily).
Fears and obsessions.
In "The Boogeyman," JJ reveals that she is afraid of the woods. When agents Derek Morgan and Spencer Reid ask her why, she tells them a chilling story about working as a summer camp counselor and discovering her boss' murdered body in his cabin. After telling the story, she admits she was joking, and says she doesn't know why she is afraid of the woods. In the same episode, JJ also states that she has a niece who, at the time, is about 8 years old. However, the canonicity of this statement is unknown, as only one sibling of JJ’s is ever mentioned, her long dead sister, Rosalyn. It could be that this niece is related to her long-term partner Will, but it is never stated that he has siblings. 
When she and Reid unexpectedly came across serial killer Tobias Hankel in the episode "The Big Game," and split up to apprehend the suspect, JJ walked into a dark barn and was attacked by several attack dogs. Although JJ shot and killed the dogs, saving her own life, Reid was abducted by Hankel in the meantime, and would later be tortured and almost killed by Hankel before the BAU team was able to rescue him. After this traumatic experience, it appears that JJ developed a fear of dogs.
In the episode "Penelope," it's announced that JJ has never shot someone until she shot the man who had attempted to kill, and who continued to threaten the life of Penelope Garcia.
She also seems to have a slight obsession with Cheetos. She can be seen eating them often, and it was implied that her online Scrabble handle was "Cheeto-breath" when she was playing Emily (who was hiding in Paris at the time).
In "Mr. Scratch," JJ tells Hotch that her worst fear as a child was being separated from her parents.