Leo Fitz

Doctor Leopold James ''Leo'' Fitz is a former S.H.I.E.L.D. agent and scientist. A genius engineer, he is one of the youngest and most prominent members of S.H.I.E.L.D. Sci-Tech division, and inseparable from his research partner and friend (and later wife), Jemma Simmons. He, along with Simmons, was recruited to Phil Coulson's Team and worked with them on all of their missions. While on the team, Fitz became good friends with Grant Ward and Skye. During the HYDRA Uprising, Fitz was an active participant in hunting for John Garrett and the rest of the Centipede Project. When Ward, a HYDRA infiltrator, was ordered to kill him and Simmons, Fitz suffered brain damage due to prolonged time without oxygen after being dropped into the Atlantic Ocean.
As time progressed, Fitz's condition seemed to get better with help and support from his fellow S.H.I.E.L.D. colleagues rebuilding it. Slowly regaining his skills, Fitz was able to get back into doing active field and lab work under Coulson's supervision, and played an important role in the conflicts with Robert Gonzales' S.H.I.E.L.D. faction and during the War Against the Inhumans. After Simmons was swallowed by the Monolith, Fitz studied the Monolith for months and with S.H.I.E.L.D.'s help, he went through a portal to Maveth and rescued her. Fitz helped Simmons research a way back to the planet, but their work attracted the attention of HYDRA, who forced Fitz to go back to the planet to bring the insidious Inhuman Hive back to Earth. Following the Battle of Maveth, Fitz helped S.H.I.E.L.D. in defeating Hive and his plan to disperse a pathogen that would turn a portion of humans into Primitives.
Since Jeffrey Mace's appointment as the new Director of S.H.I.E.L.D., Fitz worked with Holden Radcliffe on developing an Artificial Intelligence called Aida which was designed to serve S.H.I.E.L.D. as a decoy target. He was caught in an explosion with Coulson and Ghost Rider by Eli Morrow and was almost sent into another dimension, but managed to get out, and helped in stopping Morrow's evil plan. To Fitz's shock, Radcliffe betrayed S.H.I.E.L.D. and used Fitz's technology against S.H.I.E.L.D. Fitz was captured and his mind was connected to the virtual reality called the Framework where he lived a rewritten life as The Doctor, Aida's lover and HYDRA's ruthless second-in-command; he ultimately became the Framework's de facto dictator after Aida sustained injuries too serious for her to continue her duties as Madame Hydra. After he created for Aida a machine to become a real person, he was tricked by a repentant Radcliffe into approaching a backdoor to the real world; thus, Fitz left the Framework, traumatized from his behavior there. Shortly after, with the help of the Spirit of Vengeance, Fitz and his allies defeated Aida.
Despite having averted Aida's plan, it was at the cost of having become international fugitives, implicated for Mace's death, and the attack on Glenn Talbot. When his close friends were all abudcted by mysterious forces, Fitz was arrested by U.S. Air Force General Hale, who had accused him in abetting their escape. Six months into his imprisonment however, Lance Hunter rescued him and brought him to Enoch, the one responsible for the abduction of the agents. Fitz decided to go into suspended animation for 74 years to facilitate his rescue of the team.
A few months later, the ship he and Enoch were waiting on was attacked by the Confederacy, though Fitz and Enoch escaped. The two traveled through space for about a year until Fitz arrived on Kitson via the Lazy Comet and shortly after was captured by Malachi, being brought to a Confederacy ship. Upon 'reuniting' with Simmons, Quake, Piper and James Davis, Simmons bargained for her team-mates' lives and the two were plugged into the Cerebral Fusion Machine. After a confrontation with his dark self, Fitz and Simmons escaped the ship when Enoch incapacitated the Chronicoms who were watching them, with Enoch then using an Inter-Planetary Conveyance Disc to teleport them to Earth. The trip went wrong, however, and the trio found themselves on Kitson once again. Fitz was sentenced to death as he was still wanted, but the trio was saved by Izel and brought onto her crew.
FitzSimmons took the Lazy Comet back and headed for Earth, but upon realizing Izel's true nature fled from her and her crew. Fitz escaped in a Quinjet and at long last returned to Earth following a heartfelt reunion with his team. Fitz gave the agents support from the Lighthouse when they were attacking Izel, but when Chronicom Hunters attacked the Lighthouse, Fitz and Simmons were forced to retreat. With help from Enoch, Fitz and Simmons worked on a plan to prevent the Hunters to take over the Earth. To that end, they used a fragment of the Time Di'Alla to use all the time they needed, and also took some time for themselves to live some years of a peaceful life, during which Fitz fathered a daughter, Alya. Fitz then remained isolated while Simmons went to help the rest of the team travel through time and he would ensure their success by remaining hidden from both S.H.I.E.L.D. and the Chronicoms. Eventually, Fitz was reunited with his team after traveling between timelines through the Quantum Realm, and took part in the final victory against Sibyl. He and Simmons then retired from S.H.I.E.L.D. so they could raise their daughter and live a peaceful life.