WARNING: This post contains SPOILERS for Ant-Man and the Wasp.
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Marvel's villain problem. The Marvel Cinematic Universe kicked off in 2008 with Iron Man, making Ant-Man and the Wasp Marvel Studios' final film of its 10th anniversary year as well as its 20th installment. Throughout those 20 films, the MCU has received criticism for some - if not many - of its villains. Many are underdeveloped and defeated within one movie, preventing them from being further developed in future films. There are notable exceptions, like Black Panther's Erik Killmonger and the Thor franchise's Loki, as well as Captain America's Winter Soldier, who's since become an ally.
Ava was taken in by Bill Foster, who worked for S.H.I.E.L.D. at the time. In fact, S.H.I.E.L.D. built the suit Ava wears to control her molecular disequilibrium. And in exchange for S.H.I.E.L.D. working to cure Ava of her condition, which leaves her in a great deal of pain, Ava worked as an assassin for them.
Related: Ant-Man and the Wasp's Biggest Unanswered Questions
As fans of the MCU will have noticed, this backstory shares a great deal in common with Winter Soldier's. Bucky Barnes was near-fatally injured during World War II and rescued by Hydra, who healed him and gave him a vibranium arm. However, they brainwashed him and used him as an assassin, helping to further the Hydra cause without Bucky being fully conscious of what he was doing. There are a number of parallels between Ghost and Winter Soldier: Both used as weapons, both had their bodies/minds leveraged against them, both given the tools they would need by their respective organizations. Essentially, as it's explained in Ant-Man and the Wasp, Ghost is S.H.I.E.L.D.'s Winter Soldier.
Of course, we don't know how exactly S.H.I.E.L.D. used Ghost - and whether it was truly S.H.I.E.L.D. at all. After all, Hydra was intertwined with S.H.I.E.L.D. from the very beginning, up until Captain America shed light on the organization-within-an-organization. Perhaps they were the ones pulling the strings. Still, it was all under the umbrella of S.H.I.E.L.D., and there's plenty of evidence within the MCU to confirm the organization hasn't always acted heroically. Whatever Ghost did for S.H.I.E.L.D., it's clear from how Ava talks about her past that she's haunted by it, which helps to develop her character and make her a much more compelling villain.
Undoubtedly, Winter Soldier had a leg up on other MCU villains by virtue of being a brainwashed version of a character viewers already knew: Steve Rogers' best friend. Ava is first introduced and established in Ant-Man and the Wasp, which makes it more difficult for her to be developed as a truly well-rounded character, let alone a sympathetic villain. Still, despite some missteps in the final act where the movie waves a hand and seemingly cures Ava of her condition and chronic pain, Ant-Man and the Wasp's Ghost is one of the better MCU villains.
Now, since Ant-Man and the Wasp doesn't defeat Ghost by killing her, she has the opportunity to return in a future film, allowing the MCU to further develop her as a character. Whether she becomes an ally like Winter Soldier or returns to antagonizing Ant-Man and Wasp remains to be seen, but Ghost's introduction lays the groundwork for another great compelling character within the MCU.
