Ant-Man and the Wasp's mysterious villain may have a deeper connection to the eponymous characters than the trailers have led on. Considering her Marvel Cinematic Universe origin, the Ant-Man franchise's primary theme and a recent comment by star Laurence Fishburne, we think Ghost could be the daughter of Bill Foster and Hank Pym's wife Janet van Dyne.
At the Hollywood premiere of the film, Fishburne had something interesting to say about his character Bill Foster's relationship with Ghost. “I have this sort of surrogate daughter, this character called Ghost, played by Hannah John-Kamen, who is incredible,” he revealed. “We have a sort of father/daughter thing going on.”
However, so far as we know, Ghost first appeared during the events that kick off Ant-Man and the Wasp. "I actually heard what happened to you! You opened up the Quantum Realm. That's when this crazy could-be Ghost -- who, like, walks through walls and stuff -- stole your tech," Luis recounts in the film's second trailer. "Now she wants to take over the world or whatever."
The main takeaway here is that Ghost's arrival coincided with the opening of the Quantum Realm. This could be a coincidence, but coincidences rarely happen in fiction, especially MCU films. It's very possible, then, that Ghost has existed in the Quantum Realm all along, using Pym's search for Janet as a way out. If that's the case -- and it seems likely that it is -- Foster has not had a very long time to develop such a relationship with Ghost. This could indicate they have an immediate connection, and what better way to connect two strangers than to reveal one is the long-lost daughter of the other?
Should Ghost hail from the Quantum realm, her situation poses one big question: how did she get there in the first place? Well, if Janet was pregnant with her when she went subatomic, Ghost would have been born there. Janet originally entered the Quantum Realm in 1987, during a mission to disable a Soviet Missile. John-Kamen was also born in the late 80s; as such, her character is probably roughly the same age, which means Ghost could have been born that year or shortly after. Per the posters for the film, Janet aged normally during her time in the Quantum Realm, which means any potential child she could have had would also age normally.
Of course, John-Kamen's Ghost doesn't look much like Hank Pym (though she does have Janet's cheekbones). If she is indeed Janet's daughter, she would have a different father, implying she was born out of an affair. Seeing as Foster was Pym's partner back in the day, Janet and Bill would have known each other, and perhaps even developed a romantic relationship.