Jenna is an Oceanic Airlines gate attendant. She was stationed at Gate 23 - the boarding gate of Oceanic 815 - in the Airline's international departure lounge.
When Locke was waiting to board the plane she advised him that the special wheelchair that they used to load disabled passengers was not available. It was her colleague, JD, who came up with the solution of himself and Michelle carrying him on board.
Later, when Hurley arrived desperate to get on the plane, Jenna told her colleague Hunter, "this is Jenna, we've got one more" and allowed him to board, adding that it was his "lucky day". ("Exodus, Part 2")
In this scene, the season one DVD subtitles say her name is, in fact, "Gina," though her name tag still reads "Jenna."
The actress playing Jenna reappears as a desk clerk at Oxford University in "Jughead".
Judging from her accent, it is probable that Jenna is of New Zealand origin.