Captain's assistant was an unofficial Starfleet position granted to Naomi Wildman by Captain Kathryn Janeway.
The title was first thought of by Neelix, USS Voyager's chef, ambassador and morale officer, during the mission Naomi's mother went missing. Neelix came up with the title when she wanted to do more on Voyager's bridge instead of helping out in the airponics bay.
The duties of a captain's assistant included greeting visitors, arranging their replicator rations, or give them a tour of the ship, as well as delivering PADDs and getting coffee for the captain, a necessary survival activity on Voyager.
Naomi Wildman was captain's assistant for the duration of Voyager's journey home. (Star Trek: Voyager)
It became a real Starfleet position at the end of 2377 after Voyager returned to Earth. At the end of 2377, Commander Natalie Teeger became the Captain's assistant on the USS Intrepid and later the USS Intrepid-A after the Galaxy-class USS Intrepid was destroyed by a Klingon bird-of-prey in 2378. (Star Trek: Intrepid)