In the Foundation series, the mathematician who develops psychohistory is Hari Seldon. He is a mathematics professor at Streeling University on Trantor and becomes the central intellectual figure behind the Foundation project.
Psychohistory is a fictional mathematical discipline that uses the behavior of very large populations to forecast broad historical trends. It cannot reliably predict the choices of individual people, but Seldon uses it to foresee the impending collapse of the Galactic Empire and the long period of chaos that would follow.
Seldon’s response is the Seldon Plan: establish the Foundation at the edge of the galaxy and reduce the expected dark age from tens of thousands of years to roughly one millennium. His predictions lead to a political trial and exile to Terminus, where the Foundation begins.
Dors Venabili is closely associated with Seldon in the novels, but she is his wife and protector, not the creator of psychohistory. Salvor Hardin and John Hardin are also connected with the Foundation story, yet neither is the mathematician who develops the discipline.