Pfeiffer's Janet Van Dyne is the original Wasp, and the mother of Hope Van Dyne, played by Evangeline Lilly in Ant-Man and the Wasp.
In the Marvel comics, Janet is a founding member of the Avengers, and continued as a superhero even after her husband, Hank Pym (the original Ant-Man, played by Michael Douglas in the film), retired. She was eventually lost, however, in a sub-atomic realm and assumed to be dead.
Also joining the cast of the upcoming movie is Hannah John-Kamen as Ghost, Walton Goggins as Sonny Burch, Randall Park as Agent Jimmy Woo and Laurence Fishburne as Dr. Bill Foster.
Ant-Man and the Wasp hits theaters on July 6, 2018.