Seven of Nine is a fictional female Borg in the Star Trek - Voyager series. The character was portrayed on screen by Jeri Ryan. Her allegiance / loyalty is to the USS Voyager. Her fullname Borg name is Seven of Nine but most people call her just Seven.
Seven of Nine was a young girl called Annika Hensen on-board her parents research craft before being converted to a Borg machine. Her parents had been in a spaceship following and studying a Borg Cube in a non-threatening way. One day the Borg cube decided that the craft following it was a threat and decided to assimilate them.
After the two groups defeated the Species 8472, Janeway kept Seven of Nine and ordered the the Doctor to remove the Borg implants. He was able to remove all but a small number of parts, parts that allow the viewers to remember, she used to be Borg.
Harry Kim tries to romanticise Seven but she rebuffs his attempts, instead, Seven turns her attention to Doctor who is helping her to become more human. Seven develops a friendship with Naomi Wilding, a small child who is born during Voyagers travels home.