Seven of Nine
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).
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.