Borealis

Borealis is a side character in the Soldier's Memoirs saga. He is a male Thestral and the current captain of the Lunar Guard, as well as a member of both Princess Celestia's and Princess Luna's military cabinets. He is a very influential leader in the Equestrian Army and is often considered to be the greatest Lunar Guard captain of the millennium.
As of right now, not much information is known about Borealis, although it has been revealed that he has a close prior relationship with Midnight Dasher with an intensity that goes beyond superior-subordinate.