Seven of Nine was born Annika Hansen, and she was five years old when she accompanied her parents on a mission to the Delta Quadrant with the goal of studying the Borg. While the Hansens did gather some valuable data, their efforts to remain undetected eventually failed, resulting in the three of them being assimilated into the Borg collective shortly after Annika's sixth birthday. The story of the Hansen family's assimilation is told in VOY 4x06: The Raven, and the data they collected on the Borg was put to good use by the Voyager crew in VOY 5x15/16: Dark Frontier.

Seven of Nine, before and after
leaving the Borg collective, 2374
(images up-scaled using AI)

Twenty-four years later, Annika had obtained her full Borg designation, “Seven of Nine, Tertiary Adjunct of Unimatrix Zero-One”. She was selected as a liaison to facilitate communication between the Borg collective and the Voyager crew when they formed a temporary alliance against Species 8472 in VOY 3x26 & 4x01: Scorpion (Parts I and II). As part of those events, Seven of Nine was forcibly disconnected from the collective and Voyager's doctor was forced to remove most of her Borg implants. Having been assimilated at such a young age, Seven of Nine found it difficult to adapt to being human again, preferring to retain her Borg designation over her human name, though she did permit the name to be shortened to just “Seven”.

Seven was said to need “dermoplastic grafts” to help her body regenerate after having the Borg implants removed. The Doctor fashioned medicinal clothing for her to wear that would facilitate her recovery. Of course, this was the in-universe explanation for the skin-tight catsuits she wore for the out-of-universe desire to show off actor Jeri Ryan's figure.

Over time, Seven was successfully assimilated into the Voyager crew, although she still struggled with its hierarchic structure and had reservations about returning to Earth. Still, four years after being liberated from the Borg collective, Seven arrived in the Alpha Quadrant with the rest of the Voyager crew in 2378, as told in the series finale of Star Trek: Voyager, VOY 7x25/26: Endgame. After some time where Seven couldn't quite find where she belonged – including a failed attempt to join Starfleet – she found purpose in serving with the Fenris Rangers, a sort of vigilante organization that helped people in need.

Twenty-one years after returning to the Alpha Quadrant, Seven teamed up with Jean-Luc Picard in his search for Bruce Maddox in PIC 1x05: Stardust City Rag. Then, a year later, she would be transported along with Picard and several others to an alternate timeline, forced to try and repair the changes in the timeline in the year 2024, as told in Picard Season 2. Finally, in 2401, Seven would be given a field commission by Picard and then later formally commissioned by Starfleet to become the first officer of the USS Titan-A under Captain Liam Shaw, as told in Picard Season 3.