Jack is a time traveler and former con man from the 51st century. A bisexual, he is the first openly non-heterosexual character in the history of Doctor Who. In contrast to the wiser and older Doctor, Jack is more a man of action, more willing to see the hands-on solution to the problem at hand. As a consequence of his death and resurrection in the 2005 series finale of Doctor Who, the character becomes immortal, a lasting change throughout his appearances in both series. Adding another layer to the character is a vague backstory which is gradually revealed as both programmes progress.
The character's introduction served to posit him as a secondary hero and a rival to the series protagonist, the Doctor,simultaneously paralleling the Doctor's detached alien nature with Jack's humanity and "heart". In another contrast, where the Doctor is a pacifist, Jack is more inclined to see violent means to reach similar ends.
John Barrowman has described Jack as the Doctor's "muscle and guns man". Whereas in the classic series the female "companions" were sometimes exploited and sexualised for the entertainment of predominantly male audiences, the producers could reverse this dynamic with Jack, citing an equal need amongst modern audiences to "look at good looking men". John Barrowman linked the larger number of women watching the show as a key factor in this.
John Barrowman himself was a key factor in the conception of Captain Jack. Barrowman describes that at the time of his initial casting, Davies and co-executive producer, Julie Gardner had explained to him that they "basically wrote the character around [John]".John recounts Davies as having been searching for an actor with a "matinée idol quality", telling him that "the only one in the whole of Britain who could do it was you". A number of television critics have likened John's performance as Captain Jack to those of Hollywood actor Tom Cruise.
"[H]e’s bisexual, but in the realm of the show, we call him omnisexual, because on the show, [the characters] also have sex with aliens who take human form, and sex with male-male, women-women, all sorts of combinations." —John Barrowman.
Jack is bisexual, and is also the first Doctor Who character to be openly anything other than heterosexual. In Jack's first appearance, the Doctor suggests that Jack's orientation is more common in the 51st century, when mankind will deal with multiple alien species and becomes more sexually flexible.
Within Doctor Who's narrative, Jack's sexual orientation is not specifically labeled as that could "make it an issue". The bisexuality-related labels "pansexual" and "omnisexual" are also frequently applied to the character. Writer Steven Moffat suggests that questions of sexual orientation do not even enter into Jack's mind. Within Torchwood, the character refers to sexual orientation classifications as "quaint".
The character is described as both "lethally charming... good looking and utterly captivating", as well as "flirtatious, cunning, clever and a bit of an action man".
Within Doctor Who, Jack's personality is relatively light-hearted, although this changes in Torchwood's first series, where he becomes a darker character. In Torchwood Series 1, Jack has been shaped by his ongoing search for the Doctor and also by his role as a leader, in which he is predominantly more aloof. In Torchwood, he would occasionally inquire or muse about the afterlife and religion, sympathising with a man's desire to die.
Returning in Doctor Who Series 3, Jack indicates he now maintains a less suicidal outlook than before. In the second series of Torchwood, Jack became a much more light-hearted character once again, after appearances in Doctor Who where he was reunited with the Doctor.
When he observes fellow companion Martha Jones exhibiting feelings of unrequited love for the Doctor, he subtly admits he felt a degree of unrequited attraction for the Time Lord as well. Jack also appears to harbour romantic feelings for two of his employees, Gwen and Ianto, telling them both that they were the reason he returned to Cardiff, and asking Ianto on a date.
In several instances in Torchwood, Jack displays no qualms about killing a person of any species, which within Doctor Who, allows Jack's character to act in ways the lead character cannot. When reuniting with the Doctor in the 2007 series, he is verbally warned "don't you dare" when pointing a gun, and scolded when contemplating snapping the Master's neck.
Witnessing the murder of his colleague Owen Harper (Burn Gorman), Jack shoots his killer in the forehead, killing him in an act of swift revenge. By contrast, in other episodes, especially with regard to the dying, the aged, and the lonely, Jack displays kindness and sympathy, going so far as to lie to ease pain. This humanity and compassion for the dying may relate to his adamant existentialist philosophies.