Lieutenant commander
From Halopedia, the Halo wiki
Template:FactOrFiction A Lieutenant commander is 3rd in command and others are in command of departments like engineering on ships but in halo there doesn't apper to be a 3rd in commad and a Lieutenant commander is seen only twice i think, and that would be the Lieutenant commander who talked to MaKay in halo in the flood and Lieutenant Commander Fhajad.
