Premiereing in the fourth season premiere "Scorpion II," Seven of Nine is a Human female who was assimilated by the Borg and through the events of her first show became disconnected from the collective. Played by the sexy ex-soap actress Jeri Ryan, Seven (as she has come to be called) is rediscovering her humanity and proving to be most useful on Voyager's journey home.

(the following taken from Star Trek Continuum)

Species: Human, Cybernetic
Current assignment: U.S.S. Voyager : no formal assignment
Full Name at Birth: Annika Hansen
Place of birth: Tendara Colony, SD 25479
Marital status: Single

Introductory Medical Notes: EMH Doctor, AK-1

The Borg formerly known by the designation "Seven of Nine" has been disconnected from the Borg collective mind through the neutralization of the upper-spinal column neurotransceiver. In total, I have extracted eighty-two percent of her Borg hardware implants. The remaining bio-implants have been stabilized and remain critical to her life support. I have also stimulated her human metabolism and immune system, though the Borg Nanobots in her bloodstream will more than suffice until she has stabilized. Hair follicles have been repaired and stimulated. Left eyepiece has been replaced by an artificial organ replacement, simulating her own organic eye.