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. She easily qualifies as a breakout character for the series. This most likely resulted in the requirement built into her character requiring story arcs in order to introduce and build her character. Eventually she too ended up a victim of the dreaded reset button.
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 coincidences 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).
Life on Voyager
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.