Dr. Leo Fitz and Dr. Jemma Simmons were created specifically for the television series Marvel's Agents of S.H.I.E.L.D. by creators Joss Whedon, Jed Whedon, and Maurissa Tancharoen. They made their first appearance in the pilot episode, “Pilot,” which aired on September 24, 2013. The characters were conceived to fill the “Q” role from the James Bond franchise—the tech and science experts who equip the field agents. The creators intentionally designed them as a complementary pair, a “two-headed genius” who were so intellectually and emotionally intertwined that they often finished each other's sentences. This dynamic was a core concept from the outset. Scottish actor Iain De Caestecker was cast as Leo Fitz, and English actress Elizabeth Henstridge was cast as Jemma Simmons. Their on-screen chemistry became a defining feature of the series, propelling their characters from supporting scientific advisors to central protagonists whose dramatic arc would span seven seasons and redefine the show's emotional stakes. Their popularity led to their eventual, albeit brief, introduction into a Marvel comic book series based on the show, though their primary existence remains firmly within the MCU.
In the primary Marvel comic book continuity of Earth-616, Leo Fitz and Jemma Simmons do not exist. They are not established characters within the 616-canon and have no publication history prior to their MCU debut. The scientific and technological support roles within the comic book version of S.H.I.E.L.D. have traditionally been filled by a large, rotating cast of technicians, engineers, and scientists, without a specific duo matching the FitzSimmons archetype. However, due to the immense popularity of the television series, Marvel Comics launched a new series titled S.H.I.E.L.D. (Vol. 3) in 2014, written by Mark Waid. This series was designed to better align the comics with the MCU by introducing characters and concepts from the show. Leo Fitz and Jemma Simmons made their comic book debut in S.H.I.E.L.D. (Vol. 3) #1. In this continuity, they were S.H.I.E.L.D. specialists recruited by Phil Coulson. While they shared the names, likenesses, and general specialties of their MCU counterparts, their personalities and history were less developed, and this series is not considered a part of the mainstream, long-running Earth-616 history. It exists as an adaptation rather than an organic part of the prime universe.
Leo Fitz and Jemma Simmons' story begins at the S.H.I.E.L.D. Academy of Science and Technology. Both were prodigies, with Fitz specializing in weapons technology and electrical engineering, and Simmons focusing on biochemistry and biology. They met as cadets and quickly became inseparable, their intellectual synergy making them the top of their class. Their bond was so profound that their commanding officer, Agent Weaver, noted that while individually brilliant, together they were virtually unstoppable. Despite their expertise, they had very limited field experience, choosing to remain in the lab. Their lives changed irrevocably when Agent phil_coulson, recently returned from his apparent death during the Battle of New York, was tasked with forming a new, highly mobile, and specialized S.H.I.E.L.D. team. Hand-picking the duo for their unparalleled scientific acumen, he recruited them directly out of the academy. Both Fitz and Simmons, eager for a real-world application of their skills, enthusiastically joined. Stationed on the “Bus,” a modified Boeing C-17 Globemaster III that served as their mobile headquarters, FitzSimmons became the backbone of the team's operations. Fitz designed and maintained the team's advanced equipment, including the “Dwarf” drones and the non-lethal ICER sidearms, while Simmons conducted medical examinations, autopsies, and biological analyses. In their early missions, they were portrayed as naive and inexperienced in the face of true danger, often overwhelmed by the violence and moral ambiguity of espionage. Their dynamic was that of a bickering but deeply loving old married couple, a single unit that functioned best when they were together in their lab. This idyllic, contained existence was shattered by the hydra Uprising, an event that would serve as the catalyst for their painful and extensive evolution from scientists into hardened survivors.
Fitz began his career as a socially awkward, often flustered, but deeply loyal and kind-hearted individual. His intellectual confidence in the lab was contrasted by a profound insecurity in social and field situations. His entire world revolved around his work and his relationship with Jemma Simmons. This personality was irrevocably altered after he suffered severe hypoxia at the end of Season 1. The resulting brain damage caused temporary aphasia and impaired motor functions, forcing him to painstakingly rebuild his cognitive pathways. This trauma shattered his confidence and left him with lasting psychological scars, including severe anxiety and bouts of rage. His greatest psychological trial came from his time in the Framework, an alternate reality where his consciousness was reprogrammed into the ruthless head of HYDRA known as “The Doctor.” This experience fractured his psyche, causing him to occasionally hallucinate his darker half and grapple with the terrible acts he committed in that reality. Over time, Fitz became more hardened, cynical, and pragmatic, but his unwavering love for Simmons and his chosen family remained his guiding principle.
Jemma Simmons started as a cheerful, endlessly curious, and optimistic scientist who saw the universe as a series of fascinating puzzles to be solved. She was a stickler for rules and protocols, believing in order and the scientific method above all else. She was deeply compassionate but somewhat sheltered. Her experiences transformed her into a hardened and pragmatic survivor. Being stranded on Maveth, hunted by an ancient Inhuman entity, and later being controlled by that same entity (Hive) forced her to make brutal choices to survive. Her relentless, multi-year search for Fitz across space and time further honed her into a fiercely determined and sometimes ruthless individual, willing to bend any rule and sacrifice anything to save the man she loves. A major component of her later arc is the “diana” implant, a memory-suppressing device she used to protect the location of her daughter, Alya, which caused significant memory loss and emotional distress, highlighting the immense psychological toll of her journey. Despite the trauma, she never lost the core compassion that defined her.
The revelation that S.H.I.E.L.D. was infiltrated by HYDRA, and that their teammate Grant Ward was a sleeper agent, was a cataclysmic event. For Fitz and Simmons, this betrayal was deeply personal. After refusing to join him, Ward jettisoned their mobile lab pod into the ocean. Trapped at the bottom of the sea with a single oxygen tank, Fitz made the ultimate sacrifice. He engineered a desperate escape for Simmons, forcing the oxygen mask on her and confessing his feelings just before the pod imploded. While they were rescued by Nick Fury, Fitz suffered prolonged hypoxia, resulting in severe brain damage. This event marked the end of their innocence and the beginning of a long, painful road of recovery and emotional distance that would define their relationship for the next season.
While studying a Kree Monolith, a catastrophic accident occurred, and the ancient artifact absorbed Simmons, teleporting her across the galaxy to the desolate planet Maveth. Fitz was left devastated, dedicating every waking moment to finding a way to bring her back, refusing to believe she was dead. For months, he pursued every scientific theory and ancient text, alienating himself from the team in his single-minded obsession. Meanwhile, Simmons fought for survival against the planet's harsh environment and a mysterious entity (later revealed to be Hive). Fitz eventually succeeded in opening a portal and rescuing her. However, their reunion was complicated by the trauma Simmons endured and the survival bond she had formed with another stranded astronaut, Will Daniels, creating a heart-wrenching love triangle that tested their ability to reconnect.
This storyline represents the darkest chapter in Fitz's personal journey. After the team's minds are uploaded into the Framework—a virtual reality created by the LMD Aida—their lives are rewritten. In this world, HYDRA won, and Fitz is “The Doctor,” HYDRA's cold, ruthless, and sadistic second-in-command, serving alongside his lover, Madame Hydra (Aida). When Simmons enters the Framework to rescue them, she is confronted with a version of Fitz who does not recognize her and is capable of horrific cruelty. The psychological trauma of this experience, especially after Fitz realizes the atrocities he committed, haunted him long after they escaped. It created a deep fracture in his psyche, leading to a temporary split from Simmons and manifesting as a psychic projection of his evil counterpart.
After the team is transported to a dystopian future in the year 2091 to prevent an extinction-level event, Fitz is inexplicably left behind in the present. To reunite with them, he cryogenically freezes himself for 74 years with the help of the Chronicom Enoch. He awakens in the future and helps the team return to the present. However, in the finale, the Fitz who time-traveled with the team is killed in a battle with Glenn Talbot. This created a temporal paradox: one version of Fitz was dead, but another—the one still in cryo-sleep in deep space—was still alive. This discovery prompted Simmons, wracked with grief but filled with determination, to leave Earth on a grueling, multi-year quest across the galaxy to find the original Fitz, showcasing the incredible lengths she would go to for him.
The final arc reveals that after Simmons found Fitz, they spent an unknown number of years living together in peace, allowing their relationship to finally heal and flourish. During this time, they secretly designed a new method of time travel using the Quantum Realm and had a daughter, Alya. To defeat the Chronicoms, they were forced to return to the fight, but to protect Alya and the secrets of the timeline, they had to separate again. Simmons had a memory-suppressing implant, the “diana,” installed in her brain to forget Alya's location and even Fitz's face, ensuring the Chronicoms could not extract the information. The season follows a memory-impaired Simmons leading the team through time, with Fitz remaining in a secret location, guiding them from afar. Their eventual reunion in the final episode, where their memories are restored and they introduce their daughter to the team before retiring from S.H.I.E.L.D. for good, served as the emotional climax and satisfying conclusion to their epic, universe-spanning love story.
The most significant variant of Leo Fitz is “The Doctor,” his identity within the alternate reality of the Framework. Shaped by a life where his beloved father was a dominant figure and HYDRA's ideology was law, this Fitz was cold, calculating, and utterly devoid of the empathy that defined his prime counterpart. As Madame Hydra's chief scientist and lover, he was responsible for brutal experiments and interrogations. This version represents Fitz's darkest potential, a manifestation of his insecurities and a deep-seated desire for control and order. The memory and guilt of The Doctor's actions would haunt the real Fitz for the rest of his life.
A crucial distinction in their timeline involves the concept of two Fitzes existing due to time travel. The Fitz who traveled with the team to 2091 and then returned to the present was killed when a building collapsed on him during the fight against a Gravitonium-infused Glenn Talbot. However, the Fitz who entered a cryo-pod in 2017 to take the “long way” to the future was still in space, creating a divergent timeline. The team's decision to go find this surviving Fitz, rather than trying to undo the other's death, was a pivotal moment, confirming that their journey forward would be to find the “original” version of him.
As a tie-in to the television show, Leo Fitz and Jemma Simmons were adapted into the Marvel Comics continuity in the 2014 series S.H.I.E.L.D.. In this version (not considered part of Earth-616 canon), they are S.H.I.E.L.D.'s top science specialists, recruited by Phil Coulson. They assist on various missions, interacting with major Marvel characters like Ms. Marvel and the Avengers. While their core dynamic as a brilliant scientific duo is preserved, their deep, complex backstory and traumatic character arcs from the television series are largely absent. They function more as supporting characters providing tech and exposition rather than the central protagonists they are in the MCU.