Lt. Commander Usagi Tsukino is the Chief Engineer aboard the U.S.S. Benjamin Sisko. As the Chief Engineer, she falls as fifth in command of the Sisko.
Prior to her assignment on the Sisko, Lt. Commander Tsukino was an ensign, never having been able to pass any tests to qualify her for promotion to her next rank.
Having originally started out in Starfleet Culinary, the widespread usage of replicators on starships quickly made that position moot. She then moved into Starfleet security, but multiple accidents and repeated 'mishandling of firearms' demerits ended up getting her moved to Starfleet engineering where she was placed on damage control teams, mainly working on routine maintenance issues; light bulbs, carpet cleaning, things of that nature.
After about four months aboard the U.S.S. Millennium (NCC 10293), at the authority of both Admiral Genma Saotome and Admiral Happosai, she was pushed up to Lt. Commander and assigned to …