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Leopold "Leo" James Fitz is a weapons and technology expert and engineer working for S.H.I.E.L.D..
Leopold Fitz was born in Scotland, as an only child of his mother and father. Fitz' father was verbally abusive towards him by saying things like he's "not smart enough" or calling him "stupid" and "worthless" in which Jemma suspects is the reason that Fitz became so clever. Sometime around when Fitz was 10, his father abandoned him and his mother. In 2004 Fitz went to the S.H.I.E.L.D. training academy with his best friend, Biochem genius Jemma Simmons. Among their instructors were Professor Vaughn and Professor Hall, who taught chemical kinetics during their second year. However, both graduated from the academy three years early.
Sometime after they graduated from college, Jemma dragged Fitz into going on a field assignment which was to join Coulson's Team.
Leo and Jemma were recruited by Agent Coulson to be the science crew for a new team he was putting together. Stationed on the Bus, the pair has designed specialized equipment and compounds on the fly in various emergency situations.
Their first foray was the case of Mike Peterson, a man in danger of exploding thanks to the Centipede formula. Peterson was saved in part by Fitz's efforts to create a Centipede counter-agent for Peterson.
Fitz faced danger in the field in Peru, where the team was investigating an 0-8-4. Fitz and the team escaped Peruvian rebels only to face an armed takeover of the Bus. Working with the team, he helped take back the Bus and secure the old HYDRA weapon.
Soon afterward, Jemma Simmons was infected by a Chitauri helmet virus. He tried extremely hard to find an anti-serum, and he did in the end, but Jemma didn't know. Jemma hit Fitz on the back of the head with a fire extinguisher so she could jump from the 'Bus' to save the others, without Fitz following her. Fitz tried to parachute after her, but Grant Ward beat him to it. Fitz showed remorse that he wasn't the one to rescue Jemma after the events. Nevertheless, Jemma thanked him and kisses him on the cheek.
Fitz went into the field with Ward afterward. The mission was to destroy a weapon called the "Overkill Device " that was able to make weapons go off at a great distance. However, there was no extraction plan. Fitz proved his bravery in this episode over and over, ultimately helping Ward fight their enemies. Coulson and the rest of the team evacuated Fitz and Ward once they did their job.
Betrayed/near death experience
After returning from Portland, Fitz and the others find out their team is missing and Fitz later discovers a message left by Skye saying Ward is Hydra. When Simmons's autopsy report proved Ward had killed Eric Koenig, Fitz lashed out, throwing various items across the room, and had to be calmed by Coulson. After meeting Glenn Talbot at the Providence base, Fitz and the others escaped to Los Angeles where they rescued Skye from Ward.
The Team then stayed at a local hotel where they discovered Ward and Garrett are in Cuba.
While at the bottom of the ocean Fitz reveals his feelings for Simmons. Before she can respond, he hits the power button that causes an explosion which lets them escape. Using a burst of oxygen Fitz made her take, Simmons swims to the surface, dragging Fitz along behind her. They are rescued by Nick Fury.
After his very near-death experience at the hands of Ward in the final episodes of Season One, Fitz is alive, but nothing much more, judging by Simmons's state and statements to the reformed S.H.I.E.L.D. team at the Playground.
Fitz is shown to have gaps in his vocabulary and loss of certain fine motor skills because he was deprived of oxygen while escaping from the ocean (cerebral hypoxia). He is also on a regime of medication. It is revealed at the end of "Shadows" that the Simmons seen onscreen is, in fact, a construct of Fitz's imagination, since she left, in reality, to try to compel him to recover on his own.
Coulson gave Fitz the task of building cloaking technology for the bus. Fitz recognized the Overkill Device with the help of Mack. They modify the device in order to stop Creel.
When Donnie Gill, a former student of SHIELD Academy whom Fitz befriended, appeared in Morocco, Coulson and the others tried to head there before Hydra got to him. Fitz was upset that he was not going with the rest of the team. He went to the Vault D that held Grant Ward. He was shocked to see Ward in the Vault and had not been told of his presence. Ward attempted to convince Fitz that he was not trying to kill him and Simmons, and Fitz responds by turning down the oxygen levels in Ward's cell.
Reuniting with Simmons
After Simmons is compromised while working undercover at Hydra, she returns to the base. Once debriefed, she goes to greet Fitz, who she realizes has changed drastically from the happy person who he used to be.
Later, in "A Fractured House", Simmons asks Fitz to look over some documents for her, and when he tries to explain something to her, he has trouble coming up with the right words. Simmons tries to help him find the right words but fails, and Fitz asks her to stop suggesting words so he can focus. He then notices the look of pity in her eyes and asks her to stop staring at him. She misinterprets it, and questions, asking "I can't even look at you now?" He quickly corrects her saying it's not like that, and she says she's trying not to treat him any differently, to which he responds that he is different, and for some reason, she can't except that. She denies it, even though some part of it is true. He then finishes the conversation by speaking out that the one one who changed the most wasn't him, but her.
On a mission in the near future, Fitz tells Simmons that he is switching jobs to work in the garage with Mack. He then adds that the reason for the transfer is that he can work for her, just not with her.
After the incident in Puerto Rico, ("What They Become") Fitz tells Mack that he noticed Skye's heart monitor was destroyed from the inside out and thought that was unusual. ("Aftershocks") He then asks Mack how he's doing since being brainwashed, Mack tells him that he doesn't want to talk about it. Fitz replies by saying it helps to open up about things and that he knows what Mack is going through, which causes Mack to outburst by asking him angrily if he's lost control— been stuck in his own body, not being able to do anything, while he watches himself hurt the people he loves? To which Fitz replies yes. Mack then realizes that it's true and that the outburst had hurt Fitz and apologizes. Fitz forgives him saying that it's okay and Mack laments to Fitz that he hates all of the alien stuff going on.
Fitz later fixes Skye's watch and finds out that her pulse during the time of the temple collapse was almost 300 bpm. He confronts her about it, mentioning that the heart monitor was destroyed from the inside out and that she was at the epicenter of all the destruction, meaning she caused it. She starts to lose control after hearing what Fitz was saying and involuntarily makes the plane to shake with her newfound abilities. She screams in frustration and causes the lamp in her quarantine cell to shatter. Fitz becomes frightened and runs out of the room leaving Skye alone.
Skye tries cleaning up the broken glass but ends up cutting her hands on accident. May comes in and starts to notice when Simmons comes into the room as well. They are about to question her about it but Fitz bursts back into the room saying it was his fault that the lamp is broken. He also says that he checked Skye's blood sample and that it was the same as before. May and Simmons, seeming to believe his excuse, leave the room. He goes into Skye's cell and she asks him what he's done, he explains that he switched her blood sample with her old one. Confused, she asks him why, if they were different, and he tells her that her DNA has changed completely. He proceeds to clean and bandage her hand, claiming it was his fault. Fitz then suggests that they keep it between them since everyone has been on edge lately. In tears, she collapses to the floor where she is kneeling to hug him, and thank him. He consoles her, saying that she's different and there's nothing wrong with that.
In the season finale, Fitz dies while trying to save Mack and Polly. However, this is future Fitz who goes into space to save Jemma. This means that there is still a Fitz in cryo-sleep, floating around in space just waiting to be woken up in the future. This Fitz wouldn't remember being married or knowing Deke. His fate is unknown.
PersonalityFitz is socially awkward; but despite his awkwardness, he gets along remarkably well with his research partner, Jemma Simmons. He appears either jealous or very protective of Jemma when he sees her with another guy. He remains convinced that Ward can be 'recovered' long after the rest of the team consider him irredeemable. He is somewhat clumsy and awkward around Skye and sometimes Simmons.
In Season 2 Fitz has shown to become more jealous and have a lot of angry outbursts due to his cerebral hypoxia. However, the rest of the team reassures him of his improvement. In the beginning, he seems to be able to 'click' with Mack and later, Simmons.
After Fitz returns from being trapped in the Framework in Season 4, (under the illusion that he was a Hydra operative called “The Doctor”) he blames himself for everything that happened, as well as the deaths his alter ego caused, exhibiting a more serious, self loathing behavior on many occasions. He has adapted an overall darker personality after what he experienced while in the Framework.
Fitz is a Caucasian male with curly brownish blond hair, he has blue eyes, and is about 5'8". He tends to wear sweaters, overcoats, jackets, cardigans, ties, flannel shirts, and jeans. In season three you're more likely to find him in a button-up shirt and trousers, occasionally with a blazer. In the wake of his near-death experience, Fitz apparently has neural or psychosomatic issues with his right arm and hand, as is unable to use them for fine motor work. However, as Season 2 progresses, his motor senses seem to be refining with practice. By Season 3, nearly all evidence of his hypoxia seems to have been eliminated.
- Engineering: Leo is a brilliant engineer. He is a weapons/technology specialist, making gadgets to help agents in the field. He has the ability to read a machine very quickly, enabling him to dismantle and rework machine parts rapidly.
- Genius Level Intellect Fitz has an IQ around 200.
- Bilingual: Fitz is proficient in basic Arabic and can read Hebrew characters. He can also speak with a flawless American accent, an ability useful for undercover operations for a native Scot. 
- Skilled Hand-To-Hand Combat: After being in the Framework, Fitz’s skills in hand-to-hand combat has increased greatly.
- Skilled Marksman: Fitz has also gained near-expert skills with guns, from his experience in the Framework.
- Night-Night Gun & Pistol
- Full Spectrum Goggles
- D.W.A.R.F. drones
- Golden Retriever drones
- Mouse Hole
- Magic Window
- "I'm engineering, she's bio-chem."
- Fitz: "So, uh, uh, when... When you get back, I'll show you my thing. A thing. It's not-- I-It's my hardware. My equipment! Let's... hang up."
- Simmons: "You wanna make something awkward, copy Fitz."
- ―Leo Fitz and Jemma Simmons[src]
- Fitz: "Are you mental? I did explain in great detail what I meant using the Queen's bloody English!"
- Grant: "I use normal English. Words like "duck" and "run" and "might blow us to pieces.""
- Fitz: "Oh, wow! Well, congratulations, Agent Ward. You managed to string three words together in a sentence."
- ―Fitz and Grant Ward[src]
- Jemma Simmons: "I thought she was done for, but she just sweet-talked her way into Quinn's office."
- Melinda May: "How'd she manage that?"
- Leo Fitz: "Pfft, she probably just used her, um...uh...uh...her, uh... uh, boobs."
- ―Simmons, May and Fitz discussing Skye[src]
- Phil Coulson: "Any idea what could cause an effect like this?"
- Jemma Simmons: "There's the soliton hypothesis-"
- Leo Fitz: "Well, okay, judging by the horizontal-"
- Jemma Simmons: "Perhaps nano-batteries-"
- Leo Fitz: "...electrical discharge could be-"
- Phil Coulson: "Time. Let's try again. Any idea what could cause an effect like this?"
- Jemma Simmons: "Hell if I know."
- Leo Fitz: "Uh, no, no clue."
- ―Coulson tries to get an answer from Fitz and Simmons[src]
- Phil Coulson: "What's Fitz doing over there?"
- Jemma Simmons: "He detected a strange energy coming off the body."
- Phil Coulson: "He's afraid of it, isn't he?"
- Leo Fitz: "It's the smell."
- Jemma Simmons: "There's no shame in it, Fitz! It's perfectly natural to be afraid."
- Leo Fitz: "No, the only thing I'm afraid of is putrid decaying flesh corrupting my pristine workspace. (Jemma rolls her eyes) Do you remember the last time you brought a dead thing into the lab?"
- Jemma Simmons: "Oh, not the stupid cat again."
- Leo Fitz: "The cat, tell him about the cat!"
- Jemma Simmons: "And it's our lab, not yours!"
- Leo Fitz: "You left its liver next to my lunch!"
Phil Coulson: "Guys! Can we please...?"
- ―Coulson listens to Fitz and Simmons bring up old grievances[src]
- Ward: "(Disables an enemy and sees more arriving) Fitz! More Border Patrol!"
- Fitz: "(Already out of the truck and running) I'm already moving! HURRY UP!"
- ―Grant Ward and Leo Fitz on a mission[src]
- Fitz: "Good day to be a rat. I mean, there you are, minding your own business, spreading filth and disease, scavenging for grub worms or rotten fruit. When lo and behold, you see it… a prosciutto and buffalo mozzarella sandwich."
- Ward: "Fitz."
- Fitz: "With just a hint of pesto aioli."
- Grant: "Quiet."
- ―Fitz mourns his lost sandwich[src]
- Ward: "Don't say I never gave you anything."
- Fitz: "Checking for alien spectrographic signatures one teeny rock at a time."
- Ward: "Necessary precaution. We don't want anything alien getting in the wrong hands."
- Fitz: "Still, this is definitely the type of work a monkey could easily do."
- Ward: "You're our little monkey."
- ―Grant Ward and Leo Fitz while cleaning Greenwich University[src]
- Fitz: "Is Science & Technology what you imagined, Agent Ward?"
- Ward: "Yep--no uniforms, no rope course, no defined muscularity on anyone."
- Fitz: "No marching in place, no IQs in double digits."
- ―Rivalry of the S.H.I.E.L.D. Academies[src]
- Skye: "...and Fitz looks younger than us."
- Fitz: "Time will come when you won’t make fun of me for that. You’ll be jealous. You’ll be jealous wrinkly old hags."
- ―Skye and Fitz while at S.H.I.E.L.D. Academy[src]
- "Just allow me these rare moments of self-pity, okay? You're the least supportive pretend girlfriend I've ever had."
- ―Fitz, to Skye
- Skye: "Fitz, have you ever heard of an 0-8-4 being a person?"
- Fitz: "No, but I suppose it's possible. I'd hate to meet the guy."
- ―Skye fishes for information from Fitz[src]
- Sif: "She bends and shapes the will of men for her own purposes."
- May: "Only men. Her powers don't work on women?"
- Sif: "No. Men have an inherent weakness we do not share."
- Fitz: "I can't imagine what she's talking about."
- ―Sif, Melinda May and Leo Fitz discuss Lorelei[src]
"End Of The Beginning"
- Garrett: "Impressive toy."
- Fitz: "I prefer the term "hard-tech hardware.""
- ―John Garrett and Leo Fitz[src]
"The Only Light in the Darkness"
- Ward: "Everything is falling apart around us. We don't know how it'll end. If there's something you want to tell her, don't wait."
- Fitz: "Maybe Simmons should check you again for a head injury... 'cause that's not the Ward that I know."
- Ward: "You know what? Do what you want. I don't care."
- Fitz: "Good to have you back."
- ―Grant Ward and Fitz, about Simmons[src]
"Beginning of the End"
- "Ah, yeah. Well, my mum always said that you shouldn't be afraid because it's just like the way life was before you were born which wasn't that bad, was it?"
- ―Fitz to Simmons
"The Writing on the Wall"
- Skye: "How'd you two manage to acquire a body?"
- Fitz: "'Corpsey Diem' — Seize the Dead."
- ―Skye and Fitz after he and Mack retrieved Janice Robbins' body[src]
- Jemma Simmons: Best friend and colleague who eventually becomes his girlfriend and then wife.
- Grant Ward: Former friend and co-worker ("Brother figure", presumably ended by Ward's murderous attack, and Ward's recent possession by the Hive)
- Phillip "Phil" Coulson: Superior officer.
- Melinda May: Superior officer.
- Daisy "Skye" Johnson: Close friend and co-worker.
- Antoine Triplett: Friend and Co-worker (deceased)
- Lance Hunter: Co-worker.
- Alphonso Mackenzie: Friend and Co-worker.
- Deke Shaw: Grandson, with Jemma.
- Leopold: "Bold among the people," or "Bold"
- Fitz: "Son of" (in the sense of an acknowledged bastard, rather than a legitimate child)
- His last name is half of the last name "fitzsimmons", demonstrating a gestalt entity with Jemma.
- His current ID badge serial number is A 0947329; the card was issued on 5 February 2013, and will expire on 5 February 2018.
- His favorite sandwich is prosciutto and buffalo mozzarella with a hint of Simmons's homemade pesto aioli. (The Hub)
- Fitz-Simmons named their eight flying gadgets after Snow White and Seven Dwarfs. 00 is Snow White, 01 is Doc, 02 is Sleepy, 03 is Grumpy, 04 is Bashful, 05 is Sneezy, 06 is Happy (07, presumably Dopey, hasn't yet appeared on screen). (0-8-4)
- It is likely that Fitz watches the BBC TV show Doctor Who, seeing as during the episode The Hub, a miniature TARDIS (the time traveling device used by the main character of Doctor Who) can be spotted in the background as he is packing his backpack and talking to Simmons.
- Leo's real name is revealed to be Leopold. (Seeds)
- While being interrogated with the lie detector, Koenig asks him to say the first thing that comes into his mind to be in the box he found on a deserted island and he answers Simmons. (The Only Light in the Darkness)
- He claims he wouldn't know what to do if Simmons was HYDRA. (Nothing Personal)
- Fitz designed a device called a "Mouse Hole" in order to help agents cut an escape route through any material, though it was not mass-produced shortly after submitting it for approval. ("Turn, Turn, Turn") A version of the Mouse Hole cutter was used by Nick Fury himself in Captain America: The Winter Soldier, aiding him in escaping from an overturned SUV.
- Fitz believed Ward would never betray them willingly, no matter how accusing the evidence found were. (Ragtag)
- Fitz seems to have a small obsession with monkeys, an obsession which is shared by the actor who portrays him. (0-8-4)
- Fitz is getting bolder with each passing episode, willing to stand up to Garrett twice now, after being asked to join HYDRA (Turn, Turn, Turn) and telling Garrett he wants to be a big part in his downfall and after using the EMP to fry his Deathlok system (Ragtag).
- He finally admits to Simmons he's in love with her. (Beginning of the End)
- He suffered an undetermined amount of brain damage due to being without oxygen for too long. (Beginning of the End)
- According to a deleted scene of the episode Seeds , he's 23 days older than Simmons.
- He believes that his current condition (as of "Shadows") will improve, but it is unknown if it will actually get better.
- Implied that Fitz has never had a girlfriend before Jemma.