Seven of Nine was once
Annika Hansen, a
citizen of the
United Federation of Planets. Her parents conducted research on the
Borg (from a distance) aboard the
U.S.S. Raven NAR-32450, a civilian vessel. Annika lived on the Raven for three years, until the ship was detected by a
cube. The Raven crashed on a Class-M moon in
B'omar space, and six-year-old Annika was
assimilated as Seven of Nine,
Tertiary Adjunct of Unimatrix Zero One. She had no
social graces because she lived on the Raven and had next to no contact with other humans; also because she was assimilated so young. The girl was all about the
collective. At one point, when the Borg's
homing beacon was destroyed, she assimilated other Borgs by infiltrating their left cerebral hemispheres with
nanoprobes so that they would be permanently connected with a new
organic node.
Seven of Nine's first encounter with the Voyager began when she served as the Borg's representative in a temporary alliance between the Borg and the crew of the Voyager, re Species 8472. Anyway, some stuff happened, viz., an attack by Species 8472, and Seven ended up on the Voyager. She was useful too: She worked on the transwarp corridor, the new astrometrics lab, and the Delta Flyer.