From Halopedia, the Halo wiki
Ensign, abbreviated to ENS, is a junior commissioned officer rank of O-1 in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Cadet, and subordinate to a Lieutenant Junior Grade.
Initial officer posting.
- Halo: Official Spartan Field Manual - PART 8: UNSC ORGANISATION, NAVCOM Ranks, page 140-141