1. Super-Soldiers -- Jack and Gabriel
Ever wonder just why Soldier: 76 even has a number in his name? Of course not -- you're too busy handing out healing, harm, and apple slices to worry about an actual story. Plot and characterization -- outside of a few audio barks, and visual cues -- aren't exactly a focus for Blizzard's first first-person shooter. Meaning if you want to know what's what you have to dig online, and into the hints the studio left on websites and YouTube channels.
Case in point: Jack Morrison. A.k.a. Soldier: 76, a.k.a. the super-soldier turned the one-and-only dad in a game of moms. It's the super-soldier part we're interested in, though.
A fictional investigative news report on the Overwatch website explains that Morrison was once a small-town kid from Indiana. He signed up with the U.S. military for what was supposed to be a brief stint. Fortunately, or unfortunately -- depending on how you interpret Jack's self-stated status in Overwatch -- he was swallowed up by the government's worst-kept secret at the time. That is, the "soldier enhancement program."
The experiment left him with super-strength, super-agility, and the ability to bounce a shield off walls at impossible angles. Wait. Scratch that last one; we're thinking of somebody else.
Soldier: 76 wasn't the only one involved with the program, though. Which you might be able to glean from the very subtle clue hidden in his codename. We don't know for sure how many experimental soldiers there were (at least 75 more, it seems) but we do know at least one of their identities. That being one Gabriel Reyes, a.k.a. Reaper.
Reaper was once Jack's commanding officer, and lost it when Jack was appointed to command in Overwatch ahead of him. In response, Gabe (when he still was Gabe) instituted a civil war within the organization that led to its eventual loss of face with the public. As a consequence the pair duked it out so hard that Overwatch HQ exploded. With the world thinking them dead, the two took different identities. Soldier: 76 is a callback to Jack's Captain America days. Meanwhile, Reaper modeled himself after his first OC from old school notebook sketches.
2. Tracer -- Amelia Earhart Meets Doctor Manhattan
Lena "Tracer" Oxton: Overwatch agent, test pilot, box art model, and perpetually infuriating DPS-dealer. That about sums up the character's past and present. Though for her, the two periods might be one in the same.
That's because one of her earliest assignments was to test the experimental new Overwatch aircraft called "Slipstream." The jet was meant to have teleportation capabilities. It worked -- at least in the sense that it disappeared. The only problem was that nobody could find the Slipstream, or Tracer, afterwards.
That is until the pilot reappeared, months later, and only for short bursts of time. Think of it a bit like Doctor Manhattan's painful reunion with reality. Only this time the person in question wasn't granted infinite, godlike powers. She just sort of kept... Disappearing. Her molecules had suffered a "chronal disassociation" in her time away. Which meant that she couldn't stay in the present for much longer than a few days, or even a few hours.
A solution was found when one Winston came on the scene. The super-intelligent gorilla from the moon was able to craft Oxton a "chronal accelerator." What exactly that means is, of course, never properly explained. It wouldn't be very good sci-fi gobbledygook if it actually made sense, now would it?
The upshot, though, is that it allows Tracer to stay in the present for as long as she wants. Emphasis on "wants," because besides letting her experience four-dimensional reality like mortal again it gave her the power to control her own timestream. At least for up to three seconds at a time, and until Blizzard's next balance patch.
Tracer continued to use those powers both with, and without Overwatch oversight in the years that followed -- just generally fighting the good fight. Since she'd be unmoored from reality otherwise, it seems the world government let her keep her time powers after the fall of the aforementioned task force. Which might also explain why, despite decades passing and almost every other Overwatch member aging appropriately, Tracer still looks about 20 years old at the start of the game.