Seven of Nine's mind is shared by other drones.
As Voyager passes the wreckage of a Borg cube, Seven of Nine begins acting as people she once assimilated, among them a Klingon warrior and a young girl. The cause is a vinculum, the device that links drones' minds on a Borg ship, which is still working in the debris. Voyager takes it aboard and finds that it has been infected with a virus.
The virus came from Species 6339, who let some of their own people be assimilated so it could spread through the Collective and shut it down. They want the vinculum left for other Borg to find, and attack when Janeway keeps it, but Seven is at risk of being lost among the voices. Tuvok reaches her through a mind meld while Torres shuts the device down, and the vinculum is beamed away. The fifth-season episode lets Jeri Ryan play several personalities, and it ends with Seven asking Naomi Wildman to teach her a board game, Kadis-Kot.