Seven of Nine was assimilated as a child, when her parents, Magnus and Erin Hansen, studied the Borg, and were the first to encounter them directly. They were assimilated in around 2356. Her human name was Annika, and about eighteen years after she was assimilated, she was assigned to interact with the crew of the USS Voyager during the Borg-Voyager alliance against Species 8472.
Following this mission, Seven’s link to the Collective was severed, and with some assistance from the Voyager crew, she regained her individuality. When Voyager became able to contact the Alpha Quadrant, she made contact with her only known relative, an aunt, Irene Hansen.
Around 80% of Seven’s Borg implants were removed, but as she had spent the majority of her life as a drone, she was still reliant on certain implants, such as her cortical node and regeneration alcove. She was also uncomfortable using her childhood human name, and preferred to continue the use of her Borg designation of “Seven of Nine”, or simply “Seven”. She was instrumental in aiding Voyager’s journey through the Delta Quadrant and back to the Alpha Quadrant, due to the knowledge she had gained as a drone.
The Borg Queen allowed Seven’s separation from the Collective, with the purpose of allowing her to experience life as an individual. In 2375, after almost two years of life on Voyager, the Borg Queen attempted to bring Seven back into the Collective. The Queen offered Seven the opportunity to remain as an individual within the Hive, if she gave herself voluntarily to the Borg goal of perfection.
The Queen wanted Seven to assist her in assimilating of the Human race. Despite the temptation of the Queen’s offers, Seven refused to help her. Even when the Queen introduced the drone that had formerly been Seven’s human father in an attempt to persuade her, Seven refused, and was rescued by the Voyager crew.
Three years after she was separated from the Collective, Seven was contacted by Axum, who she had had a relationship with in Unimatrix Zero. He contacted her to seek her assistance in saving Unimatrix Zero. She was aware of her feelings for Axum returning during her renewed contact with him.
While her crewmates, Janeway, Torres and Tuvok, were disguised as Borg in an attempt use a pathogen to liberate the drones of Unimatrix Zero, Seven was able to make contact with the drones in Unimatrix Zero. She told the drones of the plan to destroy Unimatrix Zero. While it was still in existence, the Borg Queen would be able to trace all the liberated drones and destroy them. The liberated drones assist in returning the away team to Voyager. Seven returned to Unimatrix Zero to say a final farewell to Axum, and they were the only ones left in Unimatrix Zero when it was destroyed. Despite being in a different part of the galaxy, Axum told Seven he would find her again.
When Voyager rescued four Borg Children from an abandoned Borg Cube, Seven mentored each of them, and assisted in their rehabilitation.