Lieutenant commander

Lieutenant Commander is a junior commissioned officer rank of the UNSC Navy, equal to the rank of Major in the UNSC Marine Corps and the UNSC Army.

So far, Lieutenant Commander is the highest rank held by a Spartan-II in the case of Spartan-051 and Spartan-084. This may be due to the fact that the SPARTAN program was designed to make infantry fighters and gaining a rank higher than Lieutenant Commander would make infantry work improbable.