Kellerun Starfleet captain shaped by wartime service who was demoted and then became Michael Burnham's first officer on Discovery's last mission.
Rayner, played by Callum Keith Rennie, is a Kellerun Starfleet captain in the 32nd century. He is described as a wartime commander struggling to adjust to peace. His people suffered greatly under the Breen, and his blunt, combative style fits poorly in a Starfleet that is rebuilding.
In the fifth and final season of Discovery, Rayner commands the USS Antares when it joins the hunt for the outlaw couple Moll and L'ak. After he clashes with Federation President Laira Rillak over his methods, he is relieved of command. Michael Burnham recognizes his experience and asks him to serve as her first officer as Discovery searches for the clues to the Progenitors' ancient technology. Working with the crew teaches him to trust others and soften his approach, and he plays a central role against the Breen near the end of the season.
Rayner was the main new crew member of Discovery's final season. His arc from embittered veteran to trusted officer is one of the season's central threads.