Now that the curtain is being pulled back on Rogue One: A Star Wars Story, we know that Ben Mendelsohn's character isn't a Grand Admiral as some hoped/suspected, but a Director; specifically, he's Director Orson Krennic, an Imperial officer on the rise, who has one simple goal: be the Emperor's new right hand man.
"There is a lot of palace intrigue going on in the Empire," producer John Knoll told EW. "People conspiring to move up the ranks and sabotaging each other. There's not a lot of loyalty there."
In order to be the new right-hand man, he'd have to get past the current one, Darth Vader, who has a confusing place in the Empire, even to other Imperials.
"Vader doesn't play by the rules. He's present in the military structure, but he's not beholden to it. He's not accountable to anybody, really, except Palpatine," Kiri Hart, head of the Lucasfilm Story Group said.
Of course, we've seen others give Vader orders at time, whether it's in the pages of his Marvel Comics series when he's been placed under the charge of an Admiral or a Moff, or even when Tarkin tells him to release his Force-choke, which Hart says, "Vader does it, but not because he has to. He's just willing to give Tarkin one in that moment."
Krennic, and indeed the Empire as a whole, doesn't know just how/why Vader and the Emperor are connected (remember, as established in the canonical pages of the novel Lords of the Sith, he keeps his status as a Sith Lord a near-total secret. When he used his Force abilities in front of a village and some Stormtroopers, he had Vader slaughter them all).
Now Krennic is looking to quash this pesky Rebellion and foil their plan before Vader can (or needs to) get involved.
Rogue One: A Star Wars Story hits theaters December 16, 2016.