Harry Kim is an ensign, the most junior of post academy officers. Voyager is Harry's first posting after the academy (VOY: Non-sequitur). Kim is the primary operations officer even before half of Voyager's crew is killed, from Caretaker transcript:
KIM: Captain, I'm reading a coherent tetryon beam scanning us.
JANEWAY: Origin, Mister Kim.
KIM: I'm not sure. There's also a displacement wave moving toward us.
JANEWAY: On screen.
(The viewscreen shows a narrow band of energy heading towards them.)
KIM: It's some kind of polarised magnetic variation.
CAVIT: We might be able to disperse it with a graviton particle field.
JANEWAY: Do it. Red alert. Move us away from it, Lieutenant.
STADI: New heading, four one mark one eight zero.
CAVIT: Initiating graviton field.
KIM: The graviton field had no effect.
Recall that, on the Enterprise, Data was the primary operations officer, and he was a Lieutenant Commander, the #3 man on the ship. This is the same rank that's held by Tuvok on Voyager, who has Worf's job.
This is all fine, but what I don't get is why Kim is considered Senior Staff while only being an Ensign, and all indications are that this is Kim's first duty posting.
This gets even more confusing in The Omega Directive, where Harry is present when Janeway decides to inform the "Senior Staff" about the Omega directive, but B'Elanna is not:
Some major character officers who are higher ranking than Kim for most of the show (aside from B'Elanna's unusual absence in the Omega Directive briefing). Note that a Lieutenant is two ranks above Ensign and a Lieutenant JG is one rank above.
- Lieutenant Carey, who is definitely still alive until near the end of season 7, even though he's rarely seen after the first season
- Lieutenant Susan Nicoletti, who is still alive in Season 7
- Lieutenant Andrews, whose final appearance is in Season 3 but is never shown as being killed
Those are just the ones that get screen time. There are even higher ranking officers that appear on certain crew manifests, such as "Commander Rick Berman" in "The Killing Game" (yes, I know that Rick Berman is the name of one of the producers of the show)
So, given Harry's relatively low rank, is there an in-universe reason why he's considered Senior staff / bridge crew from basically day one?