Captain Jack Harkness is a canon character from Doctor Who and its spin-off series Torchwood. He is not to be confused with Captain Jack Harkaway, a character written by the Master of the Land of Fiction appearing in a Doctor Who novelisation.
Jack Harkness first appeared in the episode "The Empty Child" of the first season of New Who. He is a former Time Agent from the 51st century who left the Time Agency after he found out two years of his memories were missing. After some initial hiccups, he became a good friend of Rose and the Doctor. He died in a battle with the Daleks, but was brought back to life by Rose, an event which left him immortal. The latter was revealed in the first episode of Torchwood. In a later episode of Torchwood, it was also revealed that Captain Jack Harkness is an assumed name.
Because there is not much known about Jack Harkness, there is much room for speculation.
Getting It On with Canon CharactersEdit
Canonically, Jack Harkness is a big flirt and it is said that he would shag anyone "as long as they are gorgeous." In fanfic, this means that he is portrayed as an omnisexual who gets it on with pretty much everyone, a situation known as Main Character Attractiveness Disorder. In the Torchwood fandom, there seem to be two main camps: Jack/Ianto shippers and Jack/Gwen shippers.
Stories involving Ianto are often of the angsty or fluffy kind. In part, this is due to Ianto's canonical crying, which makes fanbrats think that Ianto is a woobie that needs cuddling. This more often than not results in badslash, as these authors rarely have relationship experience, much less experience having a same-sex relationship.
There also have been instances where Ianto's sex is changed (magically it seems, even though this fandom is science-based not magic-based) so that Ianto and Jack can have a heterosexual relationship.
Apart from the occasional goodfic which features hot sex, Jack/Gwen fics half the time end up (or actually begin) with Gwen getting pregnant and giving birth to a daughter. Said daughter is often a Mary Sue (see further below).
Mary Sue MagnetEdit
Jack has two types of daughters: the ones he has with Gwen, and the ones he has with another woman. Some of the latter he knows about, some he doesn't know. Without fail, when they appear in a story, his daughters are between the ages of 13 and 16.
This type of relation to Jack is not so common. This is probably because Jack is from the 51st century, or because Suethors think Jack is too old to be their brother. There are, however, those authors that manage to overcome both these obstacles: the sister is tossed into the 21st century through the Time Rift and any age difference is simply ignored.
Despite Jack being omnisexual, his former lovers are all female. Apparently, there are limits to writing slash. For some reason, his former lovers are mostly former co-workers he screwed over (in both senses of the word).
The Doctor's DaughterEdit
In a bizarre twist of fate and/or timeline, Jack ends up taking care of the daughter of the Doctor while still going up to his roof waiting for the Doctor to come back to Cardiff to recharge his batteries.
Jack and the PPCEdit
Jack can see through the perception filters the PPC agents use. It is not quite explained why he can, but he can. Though Jack himself has trouble resisting the urge to succumb to the Suefluence, seeing PPC agents does lift his spirits, and on a few occasions, he has even been helpful in holding down Sues while they were being charged.
- Jack Harkness on Wikipedia