Leopold "Leo" James Fitz was a major character in Marvel's Agents of S.H.I.E.L.D.. He was a S.H.I.E.L.D. agent who specialized in electronic and mechanical engineering, developing a number of devices for S.H.I.E.L.D.'s use. He was also the research partner and close friend - later boyfriend and eventual husband - of Jemma Simmons, the team's biochemist and medical expert. The two worked so well together that they were often regarded as two halves of the same person, with their colleagues often referring to the two of them together as "FitzSimmons".
Over the course of the series, Fitz has suffered numerous traumas during his career with S.H.I.E.L.D., both physical and emotional. He had originally established a close friendship with Grant Ward, but Ward had turned out to be a HYDRA spy and had almost killed he and Simmons after trapping them inside a containment cell that he then dropped to the bottom of the ocean. Although they both survived that incident, Fitz suffered brain damage due to going without oxygen for a prolonged period. With time and support from his friends at S.H.I.E.L.D., he would eventually recover his full cognitive and motor skills.
Fitz and Simmons' relationship was often put to the test throughout their adventures with S.H.I.E.L.D.. Rather than suffering due to mundane disagreements, the pair would find themselves facing life-threatening situations with shocking regularity, including being separated by ludicrous circumstances. In one such situation, Simmons found herself on another planet after interacting with a Monolith and Fitz would spend months trying to find a way to bring her back home. The prospect of finding Simmons still alive on a barren world after months of isolation seemed impossible to most, but Fitz refused to give up and eventually managed to reactivate the Monolith and rescue Simmons.
In Season 5, most of S.H.I.E.L.D.'s members were abducted by the Chronicom Enoch and transported to the post-apocalyptic future of 2091. Fitz, however, had been left behind and was taken into custody by the U.S. military in connection to the death of Jeffrey Mace and attempted assassination of General Glenn Talbot. After a lengthy period of interrogation, he was freed from captivity by Lance Hunter, who helped him find Enoch. Enoch explained what had happened to Fitz's friends and that there may still be a way to help them and even prevent the nightmarish future that they had been sent to. After learning the details behind Earth's destruction from the foresighted Inhuman child Robin Hinton, Fitz decided that he would rejoin his friends in the future, though he would have to take the "long way" there by simply waiting until the appointed time that they arrived on the Lighthouse. Thus, Fitz had himself cryogenically suspended aboard Enoch's ship which remained in a state of dormancy elsewhere in the solar system.
In 2091, Fitz emerged from stasis and invented a new identity for himself. Taking on the role of an alien mercenary called Boshtok the Marauder, he visited the Lighthouse where he was invited by its Kree overseer Kasius to bid on Inhumans being auctioned off as warrior slaves. While observing a battle between Daisy "Quake" Johnson and another Inhuman, Fitz dropped his cover and intervened, saving both Quake and Simmons, who had been pressed into Kasius' service. They soon reunited with Phil Coulson and the rest of their team, which now included a local inhabitant of the Lighthouse, Deke Shaw. After gathering fragments of the Time Di'alla and building a device that could activate it, the group were finally able to return to 2018, although Enoch had sacrificed himself to make it happen.
After returning to 2018, S.H.I.E.L.D. took up residence in the Lighthouse, which was originally a disaster shelter before it was converted into a space station following Earth's destruction. From here, they worked towards figuring out how the planet would be destroyed, though supposedly Quake would be responsible for the apocalypse. In the months to come, they would learn that their former ally Glenn Talbot would be responsible for destroying Earth after he absorbed a rare element called Gravitonium into his body, gaining the power to generate and control gravitational fields. Due to a combination of the Gravitonium absorption, being brainwashed by HYDRA and the brain damage he had sustained when Aida had attempted to assassinate him, Talbot had been driven insane, becoming deluded into believing that he was the world's saviour and would absorb all of the Gravitonium on the planet to ensure he was strong enough to defend Earth from the approaching threat of Thanos.
When Talbot attacked Chicago in order to extract a large deposit of Gravitonium from beneath the ground, S.H.I.E.L.D. arrived in order to evacuate civilians to safety. Fitz, Alphonso Mackenzie and Melinda May went to rescue Polly and Robin Hinton from the crashed Remorath ship that Talbot had commandeered. The ship's bow had smashed into the side of a building and so the agents evacuated everyone from the building before entering the ship. After eliminating the last surviving Remorath, Fitz, Mack and May escorted Polly and Robin out of the ship. Just as they were exiting the craft, however, a shockwave caused by the fight between Quake and Talbot shook the building. This caused a heavy pile of rubble to fall over Fitz, who was buried underneath. Although Mack and May managed to clear the debris, Fitz was too heavily wounded, with a chunk of metal piercing through his abdomen. Fitz himself did not fully realize the extent of his wounds, believing that he had simply broken his legs. While Mack and May were present, they could see that there was no way to help him in time. With tears in their eyes, they held his hand, trying to comfort him as best as they could. Fitz eventually lost consciousness and passed away.
Despite his apparent death, Fitz was actually still alive in space as this version of him had traveled back from the dystopian future, which had now been averted due to Talbot's defeat. After a heartfelt farewell to Coulson and May, who were leaving for Tahiti so Coulson could spend his final days in peace, Daisy, Simmons, Mack, Yo-Yo and Deke made plans to go into space to find Fitz and bring him back home.