Stuff I found on Reddit
The Ghost is Ava Foster, Bill Fosters daughter. Foster and Hank Pym discovered the Quantum Realm together while working for S.H.I.E.L.D., but Pym refused to continue their experiments after Janet Van Dyne disappeared. Fosters attempts resulted in the death of his wife and Ava being trapped between both realities, allowing her to phase through solid matter, but also keeping her in a state of constant pain. Foster hires Sonny Burch and his men to steal Pyms mobile headquarters so he can venture into the Quantum Realm, which he believes is the key to controlling time and space, but Ava intervenes, hellbent on destroying their research, which she blames for her mothers death and her condition.
Foster suits up as Goliath to fight Ant-Man and The Wasp when they discover he is Burchs employer and attempt to stop him from venturing into the Quantum Realm. Foster nearly defeats them before Ava kills him and then chases Ant-Man and the Wasp into Quantum Realm, where they fight. Janet intervenes, and Ava's containment suit ends up being damaged, causing her to vanish into the very fabric of reality. Ant-Man and The Wasp manage to rescue Janet before the portal collapses, but once they are reunited with Pym in the normal world, Hope and Pym vanish due to Thanos attack, leaving Scott and Janet behind.