Four of Twelve

From Memory Alpha, the free Star Trek reference

Four of Twelve, Subjuction of Unimatrix 525 was a drone in the Borg collective who had the mutation that allowed him to enter Unimatrix Zero.

In 2376 Four of Twelve was summoned to the Unicomplex in Unimatrix 01 by the Borg Queen. She asked him what he remembered about Unimatrix Zero, attempting to locate the carrier frequency that let the drone enter Unimatrix Zero. When Four of Twelve didn't reply, the Queen severed his connection to the Hive mind. The drone still said he remembered nothing. The Queen decided to have the drone dismantled and remove his cortical array (his head) for analysis instead. (VOY: "Unimatrix Zero")