Captain Kathryn Janeway

There's coffee in that nebula. - Captain Kathryn Janeway

Kathryn Janeway is captain of the USS Voyager in Star Trek:Voyager.

Canon history
Trained as a science officer, Voyager was Janeway's first command and, on her very first mission, an alien entity known as "the Caretaker" dragged her ship and crew across thousands of light-years to the other side of the galaxy. Upon his death Janeway had an opportunity to use the Caretaker's technology to send her ship home, but because it would have meant leaving a defenseless alien race open to attack, she destroyed the technology and stranded her own crew seventy-five years from home.

This is a very common theme for Janeway, and while the show's writers try to send moral messages this way (via Trek's notorious "Prime Directive"), it comes to a point that the fans (not to mention the crew) get rather sick of watching chances to get home be passed by again and again. It must be noted, though, that Janeway is an incredibly devoted captain, sometimes called a 'mother' to the entire crew; she often freely disregards the Prime Directive anyway.

Lieutenant Commander Tuvok, Voyager's Chief of Security, knew Janeway for fifteen years prior to their mutual assignment on Voyager, and they were close friends for at least half that time. Their friendship remained strong throughout the seven-year series, but was never canonically romantic.

At the time of Voyager's launch, Janeway was engaged to a man named Mark. Although Mark held out hope that Janeway and her crew were alive for a long time after their disappearance, and cared for her Irish Setter, Molly, he eventually gave up and married someone else. Janeway took this news stoically. Several episodes hint that the writers were planning to have her become romantically involved with First Officer Commander Chakotay, but nothing came of it.

In the final episode, Admiral Janeway, Captain Janeway's twenty-six-years-older self, came back through time to deliver her crew home before they suffered any more tragedies. This older Janeway was notably more cynical and pragmatic, retaining little of her younger self's idealism. The two alternate Janeways argued, and eventually came up with an everyone-wins solution that allowed Voyager to finally return home.

Within canon, Janeway is famed as having made first contact with more species than any captain since James Kirk, to which she dryly comments that "it helps to be the only starship within seventy thousand light-years". She also has quite a reputation for dealing with the Borg, and spent most of the last four seasons trying (and generally managing) to rehabilitate the former drone Seven of Nine.

She is also notably fond - to the point of dependant - on coffee.

In fanfic
In fanfiction, Janeway is often slashed with Seven of Nine, and sometimes with B'Elanna Torres. Authors who prefer heterosexual pairings often pair her with Chakotay and a few times with Tom Paris. Most of these stories have in common that they disregard the fact that as captain and commanding officer Janeway should not get (romantically) involved with any member of her crew. However, as with any type of fic, it can be very good if done well.

Janeway may also fall under the influence of a Mary Sue (e.g. using her powers as captain to give the Sue preferential treatment), but considering her strength of character, it is likely that she is at least partially resistant to Suefluence. In some stories, though, she is largely ignored by Sues or Stus because she isn't sexy enough to attract a Gary Stu's attention and she's too formidable to be the Sue's best friend. This might make her a good ally.

External link
Captain Janeway on Memory-Alpha.