From Halopedia, the Halo wiki
Doctor Catherine Halsey is the creator of the Spartan supersoldiers.
Catherine Halsey in Halo: The Television Series is portrayed by actress Natascha McElhone. McElhone was originally cast to play both Halsey and Cortana, much like Jen Taylor did for Halo 4, Halo 5: Guardians, and Halo Infinite. However, due to scheduling difficulties caused by the COVID-19 pandemic, McElhone was unable to complete her work as Cortana, so Jen Taylor was brought onboard the project to reprise her role from the games.
List of appearances