(written from a Production point of view)
This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Rebecca Alie Romijn (born 6 November 1972; age 46) is an actress and former model. She played Number One in the Star Trek: Discovery second season episodes "An Obol for Charon", "Such Sweet Sorrow", and "Such Sweet Sorrow, Part 2". 
Romijn is perhaps best known for playing the adult version of Marvel Comics character Mystique in the first three X-Men films directed by Bryan Singer and co-starring Patrick Stewart, Famke Janssen (like Romijn's father had been, a Dutch-born national), and Kelsey Grammer.