Admiral Kathryn Janeway is a version of Captain Kathryn Janeway from an alternate timeline. She appears Star Trek: Voyager's series finale "Endgame" and is portrayed by Kate Mulgrew.
In Admiral Janeway's timeline, the USS Voyager made it home to the Alpha Quadrant after a 23-year long journey. During that time, Voyager suffered the loss of a third of her crew, including Seven of Nine. Commander Chakotay would die shortly after the ship's return and Tuvok - Janeway's oldest friend and former tactical officer - became afflicted by a neurological illness that left him demented. Unable to accept the losses that she had suffered, Janeway spent the next ten years working alongside Commander Reginald Barclay at Starfleet Academy, secretly devising a plan to travel back to the past and get Voyager home early.
Admiral Janeway chose to travel to late 2377, the seventh year of Voyager's journey through the Delta Quadrant. During that time, Voyager detected elevated neutrino emissions inside a nebula that may have been a series of wormholes, but they were being protected by the Borg. Because of the Borg presence, Voyager had decided to move on, but Admiral Janeway later came to learn that the nebula contained the best means of getting Voyager home. After acquiring time-travel technology from a group of Klingon criminals, the Admiral defied Starfleet's Temporal Prime Directive and traveled back in time.
The admiral met with her past self and her old crew, offering them technology from her time that would help them get past the Borg. With a new ablative generator and transphasic weaponry, Voyager fought its way to the centre of the nebula to find their means of returning home: a Borg transwarp hub. Captain Janeway contemplated simply destroying the hub as a means of crippling the Borg by robbing them of one of their greatest tactical resources, but eventually both Janeways were able to devise a plan to destroy the hub and still get Voyager home.
Admiral Janeway puts her plan into action by flying her shuttlecraft into the transwarp hub and travelling to Unimatrix 1, the massive complex in the heart of Borg space. There, she confronted the Borg Queen, feigning an offer to exchange her technology if the Borg allowed Voyager to use the transwarp hub to return to Earth. However, the Queen was able to triangulate the position of Janeway's stealth shuttle and beamed her into the central nexus chamber of the Unicomplex. The Queen then assimilated the admiral, injecting nanoprobes into her neck.
However, before her flight, Admiral Janeway had injected herself with a neuralytic pathogen that would infect the Borg when they assimilated her. She had intended for the Queen to assimilate her, and being the hive-mind itself, the Queen transmitted the pathogen across the Collective swiftly, disrupting the hive consciousness. Janeway, in the meantime, still retained her free will and the nanoprobes injected into her left her weak and dying. She ultimately perished along with the Borg Queen as the Unicomplex collapsed around them.