Seven of Nine has served her entire adult life as a Borg drone on board one Borg cube or another. Assimilating entire worlds civilizations and their technologies without a will of her own. Only the will of the Borg collective consciousness to guide her actions. Seven of Nine was not born as a Borg she was taken and assimilated at the young age of seven.
Before she became Seven of Nine Annika Hansen explored space with her exobiologist parents Magnus and Erin. The Hansen’s were brilliant scientist that used a long range Federation Starship the Raven to follow a Borg cube through a transwarp conduit to the Delta Quadrant to study the Borg further. Using a technologies the Hansen’s developed called multi-adaptive shielding on the Raven they were able to avoid being detected by the Borg cube they were following. Their bio-dampeners allowed them to access the Borg vessels as well allowing them to gather a wealth of information.
Unfortunately for the Federation and the Hansen’s they were never able to relay the information that they learned. Due to a failure in the Ravens mulit-adaptive shielding during an ion storm 7 year old Annika and her parents were assimilated by the Borg. Annika became Seven of Nine and wasn’t seen again by another unassimilated person until the Borg attempted negotiations with Captain Catherine Janeway of the Federation Starship Voyager.
The Borg had attempted to assimilate species 8472. An alien species that was far beyond them and once war was declared species 8472 chased the Borg back to our dimension with the intent of destroying everything in the galaxy. Seven of Nine was selected to work directly with the crew of Voyager to develop weaponry to destroy species 8472.
During the war with species 8472 the Borg vessel that Seven of Nine was on was destroyed defending Voyager but not before transporting Seven and a handful of other Borg Drones on-board. The Borg eventually attempted to take over Voyager forcing Commander Chakotay to blow all the Borg out an airlock except Seven who was incapacitated. Captain Janeway decided to keep Seven and return her humanity and make her one of the crew. Ordering the doctor to remove as many cybernetic implants from her body as possible without killing her.
Annika would never reclaim her name but she would reclaim her humanity through trial and adversity. Her Borg upbringing often getting in the way of her feelings and understanding the feelings of others around her. Not to mention a massive superiority complex which, you can’t blame a drone for since they believe that becoming part of the collective is perfection. Simple things like eating a meal and much more complex social situations like dating ended with disastrously funny results. Including the dislocation of another crew members arm on her first date.
Over time Captain Janeway was able to help lead Seven back to humanity. Seven would come to put the needs of the crew ahead of her own. Even taking care of four children that Janeway would later reclaim from the Borg and leading them the way Janeway had lead her. Seven would even find love in the arms of Commander Chakotay and making Voyager her home.