According to the official character description, Captain Reyner (played by The Umbrella Academy's Callum Keith Rennie) is:
"A gruff, smart Starfleet Captain who holds a clear line between commander and crew – he leads, they follow. Rayner’s all about the mission, whatever it may be, and he doesn’t do niceties along the way; his feeling is, you get the job done and apologize later. He has a storied track record of wartime success, but in times of peace he struggles. Collaboration is not his strong suit. That said, if it serves the greater good he’s willing to learn … but it won’t be easy."
Moll and L’ak – played by Agents of SHIELD's Eve Harlow) and Shadowhunters' Elias Toufexis – come as a package:
Moll is a former courier turned outlaw, who is highly intelligent and dangerous, with an impressive strategic mind and a sharp wit. She goes into every situation with a clear plan and stays focused and clear-headed on her goal, even when things go awry. She’s not intimidated by Captain Burnham or the crew of the U.S.S. Discovery, and will face down anyone who stands in her way in order to get what she wants.
L’ak is a former courier turned outlaw, who is tough, impulsive and fiercely protective of his beloved partner, Moll. So long as he knows she’s safe, he doesn’t care about collateral damage or its consequences – a perspective that makes him very dangerous at times and will put him on the opposite side of Captain Burnham and the crew of the U.S.S. Discovery when they come into conflict.
There's no air date yet for Star Trek: Discovery Season 5.