I was partners with Hank on a project called G.O.L.I.A.T.H..
~ Foster talking about his former partnership with Hank Pym.
William Barrett "Bill" Foster is a major antagonist turned supporting character in the 2018 superhero film sequel Ant-Man and the Wasp. He is a former colleague of Hank Pym and the surrogate father of Ghost.
Bill was a biochemist who was a former colleague of Hank Pym, having worked alongside him with Project G. O. L. I. A. T. H.. At some point in time, he was introduced to a young Ava Starr, who had lost her parents when her father Elihas Starr attempted to continue his experiment in spite of being discredited by Hank for stealing his particles. Ava survived the explosion, but her body cells were gravely split in half, giving her the ability to phase through objects, but at the cost of her slowly dying because of it. Bill had since become a surrogate father for her and had spent a duration of his life finding a cure for her condition.
Bill was able to build a special chamber that feeds quantum energy into Ava to somewhat stabilize her molecules and keep her solid, however Bill discovered that Ava's illness will continue to worsen over time and the chamber can only do so much to slow down her imminent death, forcing the two to look for more drastic measures to save Ava.
Meeting Ant-Man and the Wasp
Scott Lang, Hope van Dyne, Hank Pym visit Bill looking for assistance to find Pym's portable lab which Ghost stole. Bill remains antagonistic towards Hank but offers some advice to track down the lab. The three were captured by Ghost, and Bill reveals himself to be working with her. The two plan to use the Quantum Tunnel in the lab to extract quantum energy directly from Janet so Ava can be stabilized, however, the process will most likely kill Janet. Bill is willing to take that risk as he still cares for Ava. Despite being more than willing to steal his former colleague's technology, he immediately berates Ava on her suggestion to abduct Scott's daughter Cassie.
As Lang and Hope were forced to fight off Sonny Burch and his men from stealing the lab, Ava successfully recaptures the lab and begins to perform the experiment on herself. When Bill attempts to speak her out of this, Ava rashly throws him across the room. While Ant-Man and the Wasp fight Ghost, Hank Pym confronts Bill and forces him to retreat, though he also promises to save Ava's life once Janet is rescued. Bill cautiously decides to trust Hank and leaves the shrinking lab as Hank takes the ship into the Quantum Realm.
Pym and Janet return from the Quantum Realm, and Janet heals Ava's ailment by giving her some of her quantum energy. As Ava was attempting to escape, she suggests to Bill that he leave her because he hadn't committed any crimes she had done through S. H. I. E. L. D., but Bill refuses to leave her side as he still cares for her. The two embrace before they both escape the authorities. Though it would have seemed that Bill has achieved his intent in curing Ava, it turns out that the cure is temporary as Lang and the Pym family are working on the quantum realm to find a more permanent solution for Ava's condition.
In the original comics, Bill was a superhero known as Goliath, who had the power to grow large in size (which is referenced through his mentioning of Project G.O.L.I.A.T.H., and how he had grown up to twenty-one feet in length).