Seven of Nine
Annika Hansen, better known by her Borg designation of Seven of Nine, was a crewmember on the USS Voyager for four years. Despite having a personality resembling an autistic Vulcan and a strange taste for catsuits, she is one of the more popular characters among the relatively few fans of the show, mostly because of her large breasts.
The young Annika was brought on board the science ship Raven by her parents (another example of reckless child-endangerment in the Federation), and assimilated by the Borg after the Hansens foolishly decided to spend several months flying right next to a Borg cube in an effort to study them in detail. Around two decades later, the Borg Queen came up with a typically over-complex plan to allow Seven of Nine (as she was now known) to get removed from the collective, join Voyager's crew and act as an unwitting spy for the Queen. By a miraculous chain of co-incidences this worked out, as Seven of Nine helped Voyager defeat Species 8472, had her connection to the Borg collective broken by Voyager's crew, survived surgery to remove her Borg implants after her immune system started attacking them, and agreed to join the ship's crew afterwards (although Captain Janeway gave her little choice in the matter).
Having joined the crew, Seven was able to use her vast array of Borg tricks to aid Voyager through tough times and provide resolutions to plots that the writers were too lazy to think of actual endings for. Inevitably though, any attempt to actually boost Voyager's propulsion systems failed miserably, and usually resulted in the ship nearly being destroyed. Although her best friend during her period on Voyager was the holographic Doctor, she began a relationship with Chakotay near the end of the voyage, despite the two hardly having talked previously.
While many viewers resent Seven of Nine for being an obvious attempt at trying to lure the horny male demographic, the character does nonetheless have some grudging support for being one of the show's better acted characters, and for getting some of the more interesting storylines. This often seemed to be at the expense of other characters, but in truth they had never gotten much exposure to begin with, thanks to the show's focus on aliens of the week.