First lieutenant
From Halopedia, the Halo wiki
Template:Rank Infobox First Lieutenant is a junior/company grade commissioned officer of the UNSC Marine Corps, equal in rank to Lieutenant, Junior Grade in the UNSC Navy. They command platoon sized units but can be given command of a company.
A Second Lieutenant usually receives a promotion to First Lieutenant after 18 to 24 months service.
In informal settings, First Lieutenants are referred to simply as "Lieutenant".