Not-Seven was the unofficial name for a member of a specter-like species. It was studying Rudolph Ransoms mind just before he died on the exploding USS Equinox, and it took the form of Seven of Nine due to Ransom wearing a synaptic stimulator.
Not-Seven gave Ransom the choice of dying in peace, or to live forever in an immortal dream-like state with past visions of his former lover Brianna. Ransom, feeling guilt over his actions with the nucleogenic lifeforms, chose death. (VOY short story: "The Fate of Captain Ransom")