Ptera was a Vhnori who was accidentally transported to USS Voyager after her death. The Doctor was able to revive her, but this action shook her belief in the Next Emanation, which was the Vhnori version of the afterlife.
When the Vhnori die, subspace vacuoles take their bodies to the next realm. When Ptera discovered that the vacuoles were found on a deserted asteroid, it was very difficult for her. The Vhnori believed that they would be reunited with their loved ones in the Next Emanation, and Ptera was looking forward to seeing her brother.
Ptera asked to be sent back even if it meant her death. When Voyager tried to beam her back in another vacuole, it did not work, and she died again. This time, Kes was unable to revive her, and her body was beamed to one of the asteroids. (VOY: "Emanations")