Greg "Mouse" Gerwitz is an Expert Army Ranger who served alongside Intelligence Detective Jay Halstead in the 75th Ranger Regiment. He formerly held a job at the Intelligence Unit as a civilian tech and surveillance expert before rejoining the Army Rangers in Season 4.

Biography Edit

Season 1 Edit

Mouse is first introduced in "A Beautiful Friendship" as Jay's CI, assisting him with a case. The two seem to be very close, although no one explains why or how, and he is not seen again until the next season.

Season 2 Edit

A year after his brief stint as Jay's CI, Mouse next appears in "What Puts You On That Ledge" when Jay helps him get the Intelligence tech job, putting forward his name and advocating for him to Voight, who was initially hesitant to hire him. In order to secure the job, Mouse has to meet with Platt, who asks him about his time in the military. When she questions him about his medical discharge, he becomes very clearly uncomfortable, trailing off in the middle of his answer. However, Platt lets him off the hook and gives him the job, much to the joy of both Mouse and Jay.

At the end of the episode, he thanks Jay for getting him the job, and Jay replies that Mouse was there for him, so now it's his turn to be there for Mouse. Mouse comments that he and Jay went through "the exact same thing out there" (implied to be the convoy accident Platt asked him about earlier), but Mouse fell apart, whereas Jay seemed to land on his feet. Jay tells him not to be "so sure" before Mouse tactfully changes the subject, asking if Jay is seeing anyone. Jay bemoans the difficulties of dating while being a cop, and he and Mouse toast to "plowing ahead anyway".

Season 3 Edit


In "Actual Physical Violence", Mouse is taken hostage in the lobby of the district by Jeff Frazier in an attempt to get the Chicago P.D. to find his missing daughter. He was not targeted by Frazier, instead being an unfortunate victim of opportunity, as he was the first non-civilian Frazier saw that was close enough for him to grab and capture.

He is externally very calm despite the severity of the situation, likely due to his military training; however, the police force, especially Jay, are extremely concerned about him. Mouse realizes that Frazier is an army veteran and manages to engage him in conversation using his own military service, coming to see him not as a criminal but as a desperate man looking to save his daughter. He eventually seizes an opportunity and pulls the Frazier's own gun on him, allowing the force to place him in custody. Mouse later goes to visit Frazier (who apologizes "man-to-man" for holding him hostage) and promises that he will find Sarah.

When he returns Frazier's gun to Voight as evidence, Voight notices that despite the fact that he saw ammunition in the barrel when Mouse was initially taken hostage, the gun is unloaded. This reduces Frazier's charges to reckless endangerment, meaning that he can stay with his daughter instead of going to prison. Knowing that Mouse had to have removed the bullets, he questions him about this, but Mouse refuses to admit to tampering with the evidence, maintaining that Frazier must have unloaded the gun without him noticing.

Mouse later confesses to Jay that he unloaded the gun, showing him a handful of bullets. He and Jay talk briefly about an incident during their deployment where one of their unit was kidnapped, and when they found him and his kidnappers, they did "a lot worse than hold a gun to someone's head". Mouse hides the bullets in a filing cabinet, and he and Jay drink to "extenuating circumstances".

Season 4 Edit

In "Made A Wrong Turn", Mouse confesses to Jay that he is thinking about rejoining the Rangers. Jay immediately reacts negatively, reminding Mouse of the psychological toll the war took on him. Jay then gets called away on a case, but says that he wants Mouse to talk more with him before making a decision. In the last scene of the episode, Mouse goes to see Voight and asks him for a favor, specifically for him to lie and say that Mouse's felony charge was committed as part of an undercover operation. Voight tells him he'll think about it, but doesn't offer any other information.

Three episodes later, in "A War Zone", Mouse reveals to Jay that he has received an offer from his old unit and wants to accept. He has several more arguments with Jay, eventually escalating to the point where Erin has to separate them. When Voight ignores him, Mouse snaps and has an outburst in front of the entire team, telling them that this is "the only thing [he's] ever asked for". After talking with Erin, Jay realizes that what's best for him may not be the same as what's best for Mouse and goes to Platt, who takes care of his felony charge and clears his record on Jay's request. After this, he leaves his tech job and rejoins the unit, where he currently serves.

Background and Characterization Edit

Very little is known about Mouse's family or childhood. In "Knocked The Family Right Out", he states that he did nitrous oxide in college, which he only attended for two semesters. In spite of this, he is extremely smart and proficient with tech, first shown in "What Puts You On That Ledge" when he is able to hack into Hank Voight's phone in less than a minute.

It is unknown where Mouse's nickname came from, although he does state in "What Puts You On That Ledge" that "it's a silly nickname, but I'm the first to admit it". He is almost never referred to as Greg in conversation, except by Jay in moments of high emotional tension.

Alongside Jay Halstead, Mouse served two tours in Afghanistan with the 75th Ranger Regiment, deployed to the notoriously dangerous Korangal Valley. It is revealed in his interview for the IT job in "What Puts You On That Ledge" that he (and presumably Jay) recieved a medical discharge from the army following an incident where the two were in the lead of a convoy that was bombed. In addition to being clearly uncomfortable talking about the bombing, Mouse makes several references to a traumatic experience that he and Jay went through; while it is possible that this was the bombing, neither of them have discussed it in detail and little is known about what actually happened.

Mouse is implied to suffer from PTSD from his time in the army. His behavior in Season 1 and 2 is especially indicative of this, such as drifting off during a conversation, neglecting his personal appearance, the inability to look at anyone in the eye during a conversation and nervous twitches (restless fingers and stuttering). Jay states in "Made A Wrong Turn" that following their discharge, Mouse suffered from depression and insomnia and abused drugs, also characteristic symptoms of PTSD. These gradually lessen in Seasons 3 and 4, although he still displays signs of it from time to time.

Upon his return from military service, Mouse was unable to hold down a job (likely due to his drug addiction and PTSD). He was charged with several counts of hacking and theft, including stealing a truck full of Blu-Ray players and hacking into a Department Of Defense communications satellite, although he did not serve time for either one.

Relationships Edit

Jay Halstead Edit

Jay Halstead was Mouse's best friend and part of his unit when he was deployed to Afghanistan. During Mouse's interview for the IT job in "What Puts You On That Ledge", Platt mentions that Mouse recieved a medical discharge and asks him about the incident. Mouse tells her that he and Jay were in the lead of a convoy, and although he becomes very uncomfortable and does not continue, it is implied that their convoy was bombed and Mouse and Jay were both seriously injured, resulting in their medical discharge. They leaned heavily on each other through their difficulties readjusting to civilian life and PTSD; Will tells Erin that Mouse was the one who looked after his brother after their return from war and helped him with "what he was going through", and Jay later reveals that Mouse would call him in the middle of the night when he couldn't sleep due to the psychological trauma. 

After Mouse was unable to hold down a job likely to his post-traumatic stress, he took Jay's offer of working as an informant and became his CI for a short period of time (likely due to his felony charge). The following year, Jay helps him get the job at the District, saying of Mouse, "I would trust him with my life. And I have, several times." He advocates for him when Voight is suspicious due to Mouse's criminal record and PTSD symptoms and is extremely concerned when he is held hostage in "Actual Physical Violence" to the point where Mouse has to reassure him that he is all right, and Jay even tells Voight that he should be the one to kill Mouse's captor. 

In season 4, Mouse starts thinking about going back to the Army Rangers. In "Made A Wrong Turn", he tells Jay that he's been thinking about returning to the Rangers. Jay is immediately angry and worried about him, asking if he remembers "what it did to you". He tells him that they will discuss it later, and Mouse brings it up again in "A War Zone", bringing the argument to a head. Jay also brings up Mouse's criminal record; Mouse then asks Voight for a favor to clean his record or he won't be allowed back, and Voight says he'll consider it. Mouse says he wants action, so Jay tells him to become a police officer like him, but Mouse says he was "born to be a soldier" and would re-enlist even if it meant death. Jay, blinded by his concern, refuses to let him and this angers Mouse, who then shouts back at Voight after ignoring him in front of the whole team, saying that this is the only thing he's ever asked for. Erin, after breaking up an argument between Jay and Mouse, helps Jay to realize that returning to the army is a bad idea - for Jay, and he is projecting that onto Mouse. Jay comes around and has Platt clear his record, telling Mouse at the end of "A War Zone" that if he wants to re-enlist, he's "behind him 100 percent".

In a deleted scene, he reveals that he considers Jay, as well as the rest of the Chicago P.D., to be his family, and he will always carry them with him.

Notes and TriviaEdit

Appearances Edit

Season 1 Appearances
"Stepping Stone" "Wrong Side of the Bars" "Chin Check" "Now Is Always Temporary" "Thirty Balloons"
"Conventions" "The Price We Pay" "Different Mistakes" "A Material Witness" "At Least It's Justice"
"Turn the Light Off" "8:30 PM" "My Way" "The Docks" "A Beautiful Friendship"
Season 2 Appearances
"Call It Macaroni" "Get My Cigarettes" "The Weigh Station" "Chicken, Dynamite, Chainsaw" "An Honest Woman"
"Prison Ball" "They'll Have to Go Through Me" "Assignment of the Year" "Called in Dead" "Shouldn't Have Been Alone"
"We Don't Work Together Anymore" "Disco Bob" "A Little Devil Complex" "Erin's Mom" "What Do You Do"
"What Puts You On That Ledge" "Say Her Real Name" "Get Back to Even" "The Three Gs" "The Number of Rats"
"There's My Girl" "Push The Pain Away" "Born Into Bad News"
Season 3 Appearances
"Life is Fluid" "Natural Born Storyteller" "Actual Physical Violence" "Debts of the Past" "Climbing Into Bed"
"You Never Know Who's Who" "A Dead Kid, a Notebook and a Lot of Maybes" "Forget My Name" "Never Forget I Love You" "Now I'm God"
"Knocked The Family Right Out" "Looking Out For Stateville" "Hit Me" "The Song of Gregory Williams Yates" "A Night Owl"
"The Cases that Need to be Solved" "Forty-Caliber Bread Crumb" "Kasual with a K" "If We Were Normal" "In A Duffel Bag"
"Justice" "She's Got Us" "Start Digging"
Season 4 Appearances
"The Silos" "Made a Wrong Turn" "All Cylinders Firing" "Big Friends, Big Enemies" "A War Zone"
"Some Friend" "300,000 Likes" "A Shot Heard Around the World" "Don't Bury This Case" "Don't Read the News"
"You Wish" "Sanctuary" "I Remember Her Now" "Seven Indictments" "Favor, Affection, Malice or Ill-Will"
"Emotional Proximity" "Remember the Devil" "Little Bit of Light" "Last Minute Resistance" "Grasping for Salvation"
"Fagin" "Army of One" "Fork in the Road"
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