Battalion Chief Wallace Boden
- Action Dad: He is a veteran in the CFD who holds the rank of Chief and has a step-son James. Later in mid-Season 3, his wife Donna gives birth to their son Terrance.
- Amicably Divorced: Averted. Boden is on less than amicable terms with his ex-wife Shaunda. Doesn't let that get in the way of his feelings towards his stepson James.
- Benevolent Boss: Immensely loyal and fiercely protective to any and all the firefighters and paramedics at Firehouse 51 and always looks out for their best interest, though he will take disciplinary action if he sees fit.
- Berserk Button: One should know better than to mess with the firefighters working under him or his family.
- Cartwright Curse: Openly feels this way about himself - that his relationships have a way of finding their way to being broken. He goes so far as to try to end his relationship with Donna due to being afraid of potentially screwing it up until he learns she's pregnant with his child.
- The Chains of Commanding: Due to his profession, he finds himself having to make decisions that can result in serious injury or death. The responsibility weighs heavily on him.
- Da Chief: Of Firehouse 51, Chief Boden obviously.
- Disappeared Dad: He himself is one for James, his stepson from his previous marriage, due to the divorce being so acrimonious his ex won't let him spend much time with the boy (though he and James are very close, with James calling him "Dad'). Averted with his biological son Terrance with Donna.
- Distressed Dude: In Season 4, when he's framed for aggravated assault by Serena Holmes as part of a ploy to accuse him, that he spends a night in jail, describing it as the worst night of his life before making bail.
- Experienced Protagonist: He has been a firefighter close to thirty years.
- A Father to His Men: Chief Boden, the Benevolent Boss, who always has support of his firefighters and always stands up for them.
- Good Scars, Evil Scars: Has some serious burns on his back from a fire that claimed the lives of two of his fellow firefighters twenty years back.
- Happily Married: Ties the knot with Donna in the season 2 finale. The two remain in a healthy, committed relationship.
- Heterosexual Life-Partners: With Hermann who has been with him since their Candidate days and served as his best man during his wedding to Donna.
- The Kirk: Boden is a protective Reasonable Authority Figure and the immediate superior to Casey and Severide.
- My Greatest Failure: His inability to find any survivors after traveling to the World Trade Center right after 9/11, as told in "That Day."Boden: Through all the digging and the dust and the death, we never found a single soul. And I felt like I failed them. Firefighters dont fail people. A hole formed inside of me.
- Papa Wolf: It is a bad idea to ever threaten his son or his step-son. Ask the guy who was physically abusing James.
- Parental Substitute: Boden feels this way towards Severide.
- Scary Black Man: Boden is a Benevolent Boss who genuinely cares for his men, stands up for them and is a very friendly, loyal person, but he can be terrifying if somebody gets on his bad side. Especially if they threaten his firefighters.
- Team Dad: Boden is described as "a firefighter's firefighter", being fiercely loyal and protective of all the firefighters in 51.
- Token Minority: Becomes the only black member in 51 after Mills' departure.
- We Used to Be Friends: With Benny Severide whom he served with as firefighters back in the day. The bad blood between them was because of the death of their teammate Henry Mills twenty years previously over Boden's affair with Ingrid, Henry's wife.
Lieutenant/Captain Matthew "Matt" Casey
Truck 81's commanding officer and frequently Battalion 25's Only Sane Man despite also being the character most likely to find himself caught in a high-stakes situation. He owns and operates a small construction company on his time off.
Originally engaged to Hallie, a doctor, he eventually begins an on-again-off-again (currently on, as of Season 4) with Dawson. In Season 4 he wins an election to become his district's alderman and finds that the extra responsibility and attention makes his life all the more difficult.
- Action Hero: A Chicago firefighter with the rank of lieutenant, so it goes with the territory.
- Artistic License: Casey and Dawson would never, ever be allowed to work at the same firehouse if they were in a relationship, nor would Casey be allowed to continue as her company officer. He'd be relieved of duty, as would the Chief who allowed it.
- Berserk Button: Antagonizing his fellow firefighters, his girlfriends Hallie or later Gabby. Also never bring up his Parental Issues (see below).
- Big Brother Instinct: To his sister, Christie.
- Inverted, because Matt is actually the younger of the Casey siblings (he mentions that Christie was in college when their mom shot their dad; at that time, Matt was still a teenager.)
- Big Damn Heroes
- Break the Cutie
- Butt-Monkey: Throughout season 1, Casey is put through the wringer. Ten minutes into the pilot, he loses a man on his Truck company who's embalmed right in front of him. He then under pressure to cover for a cop's son who caused a fatal vehicular accident, then repeatedly intimidated and strong-armed by said ruthless Cowboy Cop to do so. His fiancee leaves him because he wants to have a family with her. Finally, after they reconcile, she dies in a fire.
- The Chains of Commanding
- Convulsive Seizures: Casey has one in the ambulance after suffering a head injury in season 2 "Not Like This."
- The Fettered: He's rather one of the more grounded employees at 51, generally always lives by a strict code of standards, morals including testifying against Detective Voight's son that leads to their feud.
- Friend to All Children
- Firemen Are Hot: Early episodes and promotional material tended to highlight his and Severide's physiques.
- Glad-to-Be-Alive Sex: Casey and Dawson after the trying events of "I Am the Apocalypse." And that's the only reason.Dawson: This is... This is just the reaction to the stress of the day.Casey: Absolutely.
- Icy Blue Eyes: Casey's blue-ish gray eyes fit his generally aloof, cool-headed, sometimes moody demeanor.
- Jerkass Ball
- Mr. Fanservice: See Firemen Are Hot, and the audience and Dawson certainly think so.
- Number Two: In Season 6, after he is promoted to Captain, Casey is now second-in-command at 51 after Boden.
- Outliving One's Offspring: He and Gabby become pregnant at the end of season 3. In early season 4, she loses the baby due to complications.
- Parental Issues: His mother, Nancy, was incarcerated for killing his abusive father.
- Rank Up: When he's promoted to captain in Season 6.
- Red Oni, Blue Oni: Is the Blue Oni to Severide Red Oni.
- Ship Tease: With Gabby throughout Season 1 and part of season 2.
- The Spock: Generally cool-headed and approaches things with reason, in contrast to Kelly Severide.
- Vitriolic Best Buds: With Severide, though they get along well in general, they are prone to butting heads over their differing opinion.
Lieutenant Kelly Severide
Rescue Squad 3's commanding officer who takes on the most challenging assignments during calls. He's also a well-known lothario with a legendary love life.
- Beard of Sorrow: Severide briefly grows his Perma-Stubble into the months after Shay's death. He shaves it off when he returns to work at 51.
- Blood on These Hands: Severide, see Mercy Kill below.
- Break the Cutie: Kelly goes through his own fair share of loss and grief throughout the series. Lost his best friend Shay in the Season 2 finale. His new love in season 5 former cancer patient Anna who he donated bone marrow to dies anyway. Then his father who finally did something worthwhile as a father doing the firehouse a huge favor, dies from a stroke and goes down an emotional spiral.
- The Casanova: When, what season doesn't Severide catch a girl's eye.
- Crossover Ship: Two In-Universe. Severide from Fire and Lindsey from PD were in a brief relationship. He slso had a brief fling with April Sexton from Med.
- Daddy Issues: Lives in the shadow of and resents his father Benny, a philandering firefighter. Fears turning into him so much that it's his main motivation to become a bone marrow donor.
- Drugs Are Bad: Of the prescription variety - Severide was self-medicating to cover up a bad shoulder injury.
- Firemen Are Hot: Early episodes and promotional material tended to highlight his and Casey's physiques.
- Fourth Date Marriage: Severide gets married to a woman named Britney during a weekend trip to Las Vegas. They're genuinely in love and agree that they've helped each other through painful losses (Shay for him and her sister for her), but ultimately decide to end things because she has to go back home and confront her issues head on rather than stay in Chicago and avoid them.
- Freudian Excuse: Severide's inability to commit to a relationship seems to stem from walking in on his father cheating on his mother with one of his teachers. His parents all but abandoned him in the aftermath and he'd have ended up on the street if a classmate's parents hadn't taken him in. Also, he was engaged to a fellow firefighter's sister some years before the pilot. His bride-to-be cheated on him days before the wedding.
- Lady Killer In Love: In Season 5, he appears to genuinely abandon his womanizing ways when he starts an honest relationship with Anna. Later on he and Stella rekindle their spark in Season 6 and seem to make it official, with him even agreeing to take it slow with him after learning of his Fourth Date Marriage.
- Like Father, Like Son: Severide's father was a firefighter too. He was even a member of the same house and the same rescue squad.
- Long-Lost Relative: He discovers he has an adult half-sister called Katie Nolan from one of his father's affairs.
- Mercy Kill: Severide is heavily implied to have given one in the second episode.
- The McCoy: Severide is this to Boden's The Kirk and Casey's The Spock.
- Rank Up: inverted; was temporarily demoted to firefighter but was restored to lieutenant.
- Rated M for Manly
- Red Oni, Blue Oni: Brash, charismatic, and outgoing Severide is the Red Oni to the more reserved Matt Casey's Blue Oni.
- True Companions: With his roommate Leslie Shay. He's crushed by her death in the season 3 premiere.
- Vitriolic Best Buds
Paramedic in Charge Sylvie Brett
A paramedic who moved to Chicago to get away from a failed engagement. She had a brief romance with Cruz but was dumped on the night she intended to dump him.
- Adorkable: Brett is bubbly, cute and sometimes socially awkward.
- Amicable Exes: After she and Joe Cruz break up, they're still civil to one another working together with no drama. Even when Joe's feelings for her resurface
- Beware the Nice Ones: Brett is a kind, patient and loyal, but don't take it she's a pushover who won't stand up for herself. Her former childhood friend Hope found out after the
- Dude Magnet: Has an ex-fiancee, and has dated Joe, then later Antonio. In Season 7, she grabs the attention of Kyle the new CFD chaplain.
- Everyone Loves Blondes: Naturally kind and approachable, Brett gets along with everyone, just more open and soft-spoken than Shay.
- Fish out of Water: When she first appears working as a paramedic who's moved from a country hometown to the urban city of Chicago.
- Hair of Gold, Heart of Gold: She's bubbly, optimistic and kind-hearted, and a blonde to go with it.
- Happily Adopted: She reveals this to Dawson in Season 5, though she would really appreciate it if her birth parents would at some point seek her out.
- Heterosexual Life-Partners: With Gabriella and Chili. In Season 7, she starts to develop this with new partner Emily Foster.
- Hidden Depths: Sylvie reveals a talent for literature, in helping Mouch write his secret firemen based romance novel.
- Innocent Blue Eyes: Portrayed by Kara Killmer.
- Ms. Fanservice: She's rather cute. Seen in her underwear in season 3 episode 5 ... then later to Antonio's pleasure when they start dating.
- Red Oni, Blue Oni:
- Runaway Fiancé: In her introductory episode, Brett lays it out to Otis that she ran out on her Childhood Friend fiancé Harrison. She stared at the prospect of marriage and "couldn't pull the trigger." Subverted as its revealed Harrison was the one who called off the wedding.
- Single Woman Seeks Good Man: Twice so far in the series. First she goes out with Joe Cruz who she says is "the nicest guy in the world." Later she's shown to be crushing on Antonio and they start dating in Season 5.
- Suspiciously Similar Substitute: A blonde paramedic who replaces Shay as Gabby's partner. Severide is startled when he sees Brett for the first time when Ambo 61 goes into service, seeing her blonde hair and thinks it's Shay before Brett turns to face him.
- Too Much Information: She can't help but go into extreme detail of her and Antonio Dawson's love life. Her unfortunate audience is usually Gabby Dawon, Antonio's sister.
- Why Did It Have to Be Snakes?: She does not like clowns, so is quite humorously squeamish when treating a patient wearing clown makeup.
Firefighter/Lieutenant Christopher Herrmann
Casey's second in command and Truck 81's official relief commander and source of a lot of the mischief the crew gets into; now the lieutenant in command of Engine 51. He fancies himself as a master businessman despite most of his schemes backfiring on him one way or another with some having long-term consequences.
He's part-owner of Molly's, a neighborhood bar and firefighter/cop hangout, along with Dawson and Otis.
- Action Dad: Has four (later five) kids of his own and a firefighter.
- Berserk Button: Neglectful or abusive parents, as Mouch quoted "Flips his switch".
- Deadpan Snarker: Along with his smart mouth, Hermann has a very dry, snarky sense of humor.
- Experienced Protagonist: Has been a firefighter for nearly as long as Boden.
- Family Man: Has a stable close-knit family being Happily Married to his wife Cindy with their four sons and one daughter.
- Friend to All Children: Has a soft spot for young victims. Especially since he's a father with kids.
- Get Rich Quick Scheme: Hermann spends the first season going through a number of them to try and make enough money to buy his family a house after they were evicted in the series premiere. None of them work out and his wife eventually talks him out of pursuing more of them. He eventually finds a steady side business in running Molly's a neighborhood bar that quickly becomes a firefighter/cop/doctor hangout.
- The Heart: Casey says Hermann is this for firehouse 51 himself.
- Heterosexual Life-Partners: With both Boden and Mouch.
- Hot-Blooded: Very blunt, outspoken and known for losing his temper and shouting.
- One of his schemes ends up having lasting career ramifications as he'd pissed off a chief on the promotion board and the chief then made it his mission in life to block Hermann's promotion to lieutenant.
- Jerk with a Heart of Gold: Hermann is headstrong, opinionated and snappy, has a bad temper to go with his snarky humor. Nonetheless he is a Family Man for his wife and kids, and is T He Heart of all of Firehouse 51.
- The Leader: In Season 7, Hermann becomes lieutenant and becomes the commanding officer for Engine 51.
- Massive Numbered Siblings: His wife gives birth to kid number five early in the series... who was conceived the first time he made love with his wife after several months (he takes out his frustration by pounding a limousine with a halligan bar).
- Number Two: Hermann acts as Casey's de facto right hand, taking charge of Truck 81 when Casey is unavailable. This becomes more set in stone when Hermann passes the lieutenant's exam and becomes the relief commander of the company.
- Ponzi: Herrmann falls victim to one of these until Otis fakes being a lawyer and gets him out of it.
- Rank Up: Boden promotes him to lieutenant just to infuriate Jerry Gorsch.
- Skewed Priorities
- Shirtless Scene: Hermann rips off his shirt in triumph (and reveals he's in excellent shape) when Casey complains that the older man has a lower body fat percentage than him.
- Sitcom Arch-Nemesis: In season four, Hermann develops an antagonistic relationship with a hipster family from Portland, Oregon who shoot down every one of his attempts to be friendly and try to close down Molly's. He begins retaliating by being as annoying as possible.
Firefighter Randy "Mouch" McHolland
A veteran on Truck 81 who, along with Hermann, is the source of most of the house's harebrained schemes. When not on calls, he's usually found on the couch in front of the TV hence his nickname "Mouch" (part man part couch). Starting in Season 3 he's dating, and eventually marries, Trudy Platt, CPD District 21's desk sergeant.
- Ascended Extra: Recurring character in Season 1, before being promoted to main.
- Bunny-Ears Lawyer: Mouch typically serves as union representative for when the crew at 51 face disciplinary action or a lawsuit and very knowledgeable in the legal and political side of the CFD.
- Characterization Marches On: According to show co-creator Derek Haas, Mouch originally was "this lay-about, sort of old guy who's just trying to ride out his last couple of years in the firehouse."
- Disappeared Dad: An unintentional example. He'd donated sperm many years before and promptly forgot about it but his grown biological daughter eventually tracks him down after his donor information was accidentally leaked.
- Experienced Protagonist: Mouch is a veteran firefighter, having served as long as Hermann and the longest-serving member of 51.
- Embarrassing Nickname: He tries to get his buddies to stop calling him "Mouch" when he makes a run for the CFD Union presidency, believing that a nickname highlighting his laziness would hurt his election chances. His buddies, being who they are, refuse to do so.
- Happily Married: To Trudy Platt as of late season 4.
- Heterosexual Life-Partners: Very close to Hermann.
- Idiot Ball: Trudy, his wife, has this opinion of him and he usually agrees. Depending on her mood, it can be affectionate or disparaging.Trudy: This one...he's an idiot!
- No Social Skills
- Only Known by Their Nickname: So much so that "Mouch" rather than McHolland is the name stitched on his turnout coat. Trudy, his wife, is the only person who regularly addresses him by his given name.
- Retirony: had a heart attack during what he'd intended to be his final shift. Surviving it convinced him to stay on.
- Unmanly Secret: He's written a series of firefighter-themed romance stories but the department catches wind of it and threatens to fire him if he has any of them published.
Firefighter Joe Cruz
Originally Truck 81's chauffeur, he transfers to Squad 3 to fill its constantly vacant spot after secretly obtaining Rescue Squad Certification from the CFD Academy. He's roommates with Otis and a very popular Zumba instructor on the side.
- Ascended Extra: Got promoted from recurring character to principal cast in season 2.
- Beware the Nice Ones
- Big Brother Instinct: He's very protective of his younger brother Leon.
- Blood on These Hands: He left a gangbanger Flaggo to die in a fire, a gangbanger that was grooming his brother to join his gang.
- Death Seeker: Cruz was this for a while, after the Blood on These Hands incident above. Fortunately, an intervention by Mouch and Casey sets him straight.
- Heterosexual Life-Partners: With Otis.
- Murder by Inaction: Cruz in season 1, leaves a victim to die in a fire, the "victim" being the boss of his brother's gang. Justified as Cruz was only trying to protect his brother.
- Omniglot: Of Latina descent and fluent in Spanish
- True Companions: He's this with Otis, being roommates.
- Un Manly Secret: His side profession as a Zumba dance instructor that comes out in Season 3.
Firefighter Stella Kidd
A new firefighter assigned to Truck 81 upon the closure of her former firehouse. Stella is revealed to have history with Gabriela Dawson and Kelly Severide.
- Awful Wedded Life
- Brutal Honesty
- Deadpan Snarker
- Heterosexual Life-Partners: Has this dynamic with Dawson.
- Love Makes You Stupid: In Season 7 episode "Going to War" when trying to extinguish a fire trapping Severide, she runs out of air, but ignores her low air alarm and collapses, nearly dying. Casey calls her out on this in a later episode, standing by an order she cannot work a call where Severide's life is on the line.
- Raven Hair, Ivory Skin
- Remember the New Guy?
- Ship Tease
Paramedic Emily Foster
An easy-going and confident paramedic and is a brand new to the CFD, and Brett's new partner on 61.
Former Main Characters
Dr. Hallie Thomas
- Brainy Brunette: She works as a physician at Lakeshore hospital.
- Character Death: In the season 1 episode "Leaders Lead".
- Child Hater: Downplayed as Hallie never shows any real dislike towards kids, she just doesn't want to have kids. It's one of the reasons that drives her and Casey to split up again.
- Hospital Hottie: Hallie is quite an attractive woman and is a doctor.
- Killed Off for Real
- Nice Girl
- Put on a Bus
- Relationship Revolving Door: Has one with Casey.
- Satellite Love Interest: She has little to no relevance to the plot apart from being Casey's on-again/off-again fiancee.
Paramedic Leslie Shay
- All Lesbians Want Kids: A recurring subplot throughout the first season is paramedic Leslie Shay's quest for a child of her own. At one point, unable to afford artificial insemination, she's desperate enough to ask her best friend Kelly Severide to impregnate her the natural way.
- Back for the Dead: Lauren German makes one final appearance, credited as a guest star, as Shay in the Season 3 premiere.
- Blue Oni: Cool-headed and humorous, to both Dawson and Severide's red.
- Break the Cutie: In Season 2 episode 4, a patient commits suicide right in front of her, her best friend come to harsh words with her over it and sends her into a downward spiral.
- Character Death: In the Cold Opening of the Season 3 premiere and much of the episode deals with the impact it has on Firehouse 51.
- The Confidant: Shay is this for Dawson.
- Everyone Loves Blondes: Shay is loyal, supportive and well-liked by her co-workers, especially her partner Dawson and best friend Severide.
- Heroic BSoD: In episode 4 "A Nuisance Call", a patient on a routine call commits suicide in front of Shay and Dawson, and hits Shay pretty hard.
- Heterosexual Life-Partners: With Gabriella and Severide, despite being female to her and gay to him.
- Lipstick Lesbian: When off-duty at least.
- Killed Off for Real: In Season 3, it's revealed the Season 2 finale's Cliffhanger resulted in her death, and much later that it was the work of a serial arsonist.
- Straight Gay
- Stuffed In A Fridge: Shay's death seems to exist only to cause Severide and Dawson pain. Nothing in her own story lead up to it.
- Sugar-and-Ice Personality: Generally aloof, cool-headed and snarky, but has a warm hearted, supportive and well-liked at 51.
- Undying Loyalty: To all of her friends, especially Kelly, to the point she's willing to supply him with painkillers when his arm was hurt, and to forge a urine sample in order to help him pass a drug test, however she does eventually get tired of it and moves out until Kelly decides to get help.
Firefighter Brian "Otis" Zvonecek
Originally Truck 81's aerial operator (and nicknamed "Otis" after the elevator company as a result), he eventually becomes the company chaffeur after Cruz transfers to Squad 3. A proud Russian from a huge family, he lives with his grandmother and Cruz. He is part owner of Molly's with Hermann and Dawson. In the season 8 premiere he "Sacred Ground", he is badly burned in a fire, later dying in the hospital of his injuries.
- Ascended Extra: Appeared in season 1 in a recurring capacity until promoted to main character in season 2.
- Cool Uncle: Has three nephews who visit him in "The Nuclear Option", and he's excited to give them a ride on the Truck.
- Desk Jockey: Placed on medical leave after being shot in the neck a bullet set off by a house fire, Otis decides to help out around the firehouse, taking a desk job.
- Embarrassing Nickname: He asks that the firehouse stop calling him "Otis" once he's no longer a candidate but his request is quickly shot down because nicknames are forever. When a relief chief asks Mouch how to pronounce Otis's last name Mouch bluntly replies "O-Tis", over emphasizing each syllable.
- Fan Boy: likes Settlers of Catan and Battlestar Galactica (1978) to the point of owning a Colonial warrior's flight helmet autographed by Richard Hatchnote , dislikes Battlestar Galactica (2003).
- Heterosexual Life-Partners: With Cruz.
- Killed Off for Real: In the season 8 premiere, he is badly burned in an explosion and later dies in the hospital.
- Momma's Boy: Or grandmother's boy. He has his grandmother ("Baba") move to Chicago to live with him and, although Cruz finds it strange and uncomfortable, even shares his room with her, having purchased bunk beds for the purpose.
Firefighter/Paramedic Peter Mills
- Amicable Exes: With Gabriella.
- Berserk Button: Do not bring issue of his father who died working as a firefighter or badmouth him as Benny Severide found out the hard way.
- Broken Pedestal: For Chief Boden and Gabriella when he learns that Boden once had an affair with his mother and that Gabriella had kept it secret from him which along with her obvious feelings for Casey leads to them breaking up.
- Butt-Monkey: Played for Laughs, in early season 1, when he starting as the new candidate so the firehouse hazing process begins. In the pilot, Dawson lets him think Shay is interested in him, only for her to later tell him she's gay, humiliating him. Then at the annual CFD barbecue in episode 2, he's told to show up in his 'dress blues', only for him to be the only one who does so.
- Career-Ending Injury: He suffers a head injury that causes severe balance issues, effectively ending his career as a firefighter (although he sticks around as a paramedic). Subverted towards mid-season 3 when he's cleared for duty again
- Full-Name Basis: The firefighters of Truck 81 developed a habit of always calling him "Peter Mills" while he's a candidate.
- In the Blood: Peter Mills' father was a firefighter who died when Peter was a kid.
- Long-Lost Relative: He never knew his father's family; his mother told him they moved to Seattle, and sets out to find them in Season 3.
- New Meat: In Season 1, he's employed at 51 as the new firefighter candidate on Truck 81, and is immediately the target of the firehouse's humor.
- Nice Guy
- Put on a Bus: He is given a proper send off in Season 3. He decides that being near his family is worth more than the thrill of being a firefighter so he moves to North Carolina.
- Real Men Cook: He immediately gets on the good side of Firehouse 51 with his cooking. He also volunteers at his family's restaurant.
- Token Minority: The only black member on Truck Co. 81.
Paramedic Jessica "Chili" Chilton
A new paramedic on Ambulance 61, hired to replace Peter Mills.
- Cynicism Catalyst: Her twin sister Alyssa aka Jelly Bean, a CI of Detective Antonio Dawson is found dead which leads to her heavily drinking and spiraling out of control and her relationships with her co-workers suffering.
- Heterosexual Life-Partners: With Sylvie Brett.
- Parental Issues: According to her, her parents didn't get along .
- Put on a Bus: She is fired by Boden because of her drinking problem and her resulting behavior.
- Quirky Curls: Chili has these.
- Ship Tease: With candidate Borelli. They eventually begin a liaison which ends after her behavior starts to get to him.
- You Look Familiar: had previously played her character's sister on Chicago P.D..
Paramedic Jimmy Borelli
Originally a Candidate on Truck 81, Jimmy found himself bumped off when house closures resulted in a more experienced firefighter transferring to House 51. Chief Boden arranges for Jimmy to stay at the house as a paramedic until a more permanent firefighter slot opens up for him.
His career is abruptly cut short when he carelessly got too close to a chemical fire and literally had half of his face burned off.
- Broken Pedestal: Loses all respect for Chief Boden after Boden's was forced to leave his brother to die while pinned under wreckage of a building about to collapse. Preventing the unit going in a pointless attempt to remove tons of breakage that pinned him down. While the building was a minute away from collapsing which it later did. While Jimmy despised Boden for making that choice and saving him despite himself when he refused to leave his brother behind. None of the rest of the squad did so, supporting Boden's command decision.
- Eye Scream: His left eye is lost after he reckless attempt to save a victim from a burning car in the second episode of season 5.
- Leeroy Jenkins: Ignores Boden's orders in an attempt to save a victim trapped in burning car. His recklessness deprived him the use of his left eye after suffering severe burns to the face and neck.
- Firemen Are Hot: Virtually the case when he started as a fireman candidate before being the firehouse paramedic.
- New Meat: His career as a firefighter didn't get off to a great start. He's fired on his first day (due to a prank his older brother pulled), has to beg for a second chance, and eventually loses his spot because a more experienced firefighter transferred to House 51.
- Rebuilt Pedestal: After Jimmy almost loses his life and half his face after refusing to leave a person trapped in a car and about to die. Despite Boden ordering him to do so as it was about to explode. He accepts solace from the Chief finally understanding the reasoning behind Boden's actions regarding leaving his brother behind to die.
- Single Tear: Sheds one out of his remaining eye as Chief Boden stays by his side in the hospital.
Paramedic/Firefighter Gabriella Dawson
Originally a paramedic studying to become a doctor. She drops this goal and decides to pursue a career as a firefighter instead. She spends most of the early seasons pining after Casey and feeling jealous due to his relationship with Hallie. She and Casey eventually get together but repeatedly swing from hot to cold. In Season 5, she returns to being a paramedic, due to her fearing for her own safety after taking in a foster child.
- Action Girl
- Amicable Exes: With Peter.
- Artistic License: Casey and Dawson would never, ever be allowed to work at the same firehouse if they were a couple, due to the potential conflict of interest. If that happened in real life, he would be relieved of duty and demoted, same for the Chief who allowed it to happen.
- Dawson's insubordination and disregard for CFD regulations would result in her being fired and prosecuted in real life.
- Chronic Hero Syndrome
- Genre Savvy: She's been with the CFD for several years before becoming a firefighter so she knows how to avoid most of the pranks Candidates have to endure. The rest of the house has to work extra hard to catch her by surprise.
- Heterosexual Life-Partners: With Leslie Shay, despite Shay being most definitely not heterosexual.
- Hot-Blooded: Dawson is very impulsive and stubborn and it gets her into trouble in her profession on more than one occasion.
- I Did What I Had to Do
- It's All About Me: Makes major life-changing decisions without any input from Matt, or even consideration for Matts feelings on the matter. It comes to a head when she wants to get pregnant again. Matt is understandably concerned about the risks due to her previous ectopic pregnancy, but she gets pissed at him for researching adoption instead. When she declares, "My body, my risk, my choice," Matt finally gives her a long-overdue What the Hell, Hero?. Rather than admit that she could be wrong, Gabby decides to fly to Puerto Rico.
- It's All My Fault: She feels this way about Shay's death, because in the building explosion, she'd switched places with Shay for her to give rescue breaths on a victim which put Shay directly in the blast area.
- New Meat: Despite having served at Firehouse 51 for several years, Dawson is bumped to the bottom of the house's pecking order once she becomes a Candidate.
- Nice Job Breaking It, Hero!
- Outliving One's Offspring
- Parental Substitute
- Put on a Bus: left to assist in the post-Hurricane Maria reconstruction of Puerto Rico.
- Red Oni, Blue Oni
- Ship Tease
- Single Woman Seeks Good Man: Gabriela often comments on Casey's qualities and how those are major attraction points for her.
- Spicy Latina: Has a quick temper, very outspoken, passionate and stubborn and is of Latina descent.
- What the Hell, Hero?
Firefighter Harold Capp
- Character Development: Originally just a recurring extra. He's developed into a minor source of comic relief around the house.
- Hollywood Tone-Deaf: Fancies himself a singer, having been in an a capella group in high school. Everyone else finds his singing to be simply dreadful.
- Number Two: On Squad 3 after Severide.
- Recurring Extra: He's present in nearly every episode (and occasionally pops up in spin-offs) but he rarely speaks.
- Those Two Guys: He and Tony act as this for the other more central members of Squad 3.