Iko was a Nygean with a history of violent behavior, including the unprovoked murder of a young father. In 2377, he was due to be executed on the Nygean homeworld, but the prison ship transporting him suffered a critical accident. Iko and the others were transported to the USS Voyager.
Upon his arrival on Voyager, Iko held Seven of Nine hostage, but was tricked into switching her for The Doctor, not realizing he was a hologram. He was subsequently stunned by Tuvok and taken to the cargo bay. Later, he was severely beaten by the warden Yediq and his guards after threatening Yediq's family, which caused a severe edema in his parietal lobe. The Doctor was able to bypass the edema using Seven's nanoprobes, but as a side effect the nanoprobes repaired a congenital birth defect: a disconnection of a node analogous to the Human pineal gland, responsible for regulating behavioral impulses and decision-making.
After the procedure, Iko's behavior changed completely and he came to experience remorse for the first time in his life. The Doctor asserted that the neural defect responsible for Iko's earlier sociopathy had been corrected, and thus his death sentence should be reevaluated. However, Yediq was skeptical and Iko himself refused, believing that he deserved to die for what he did. In the meantime, Iko became friends with Seven, who had a vested interest in the success of his reformation due to her own atrocities committed during her time as a Borg. She found in common with Iko an interest in astronomy.
During the attempted break-out engineered by Joleg and his brother, Joleg took Yediq hostage. Feigning a desire for revenge against the warden, Iko was able obtain a weapon and stop Joleg from killing Yediq. This convinced Yediq that Iko had indeed changed, prompting him to influence the family of the victim to hear Iko's appeal. His appeal was nevertheless rejected, and he was presumably executed shortly after. (VOY: "Repentance")