Jack Harkness

Captain Jack Harkness is a canon character from Doctor Who and its spin-off series Torchwood.

Canon History
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.

In Badfic
Because there is not much known about Jack Harkness there is much room for speculation.

Getting it on with canon characters
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 Torchwood fandom there seem to be two main camps: Jack-Ianto shippers and Jack-Gwen shippers.

Ianto
Stories involving Ianto are often of the angsty or fluffy kind. In part this is due to Ianto's canon 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 was changed (magically it seems, even though this fandom is science based not magic based) so that Ianto and Jack could have a heterosexual relationship.

Gwen
Apart from the ocassional 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 a Mary Sue, see further below.

This situation inspired agents Emma and Tasmin to gather the Torchwood Crew together in their briefing room and brief them on the proper use of condoms.

Daughters
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. These daughters almost invariably have the surname "Harkness," even those who were born before Jack adopted that name.

Sisters
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.

Former Lovers
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 Daughter
In a bizarre twist of fate and/or time line 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.