The Companion was an energy cloud in the Star Trek episode "Metamorphosis" that kidnapped Zefram Cochrane, rejuvenated him, and kept him alive for 150 years. It could create food and travel at warp speed through open space.
The Companion was in love with Cochrane, and wanted him to be happy, so it kidnapped a shuttlecraft containing Kirk, Spock, McCoy, and the Woman of the Week, so Cochrane would have friends. To keep the new arrivals from leaving the companion used telekinesis, pyrokinesis, and teleportation.
Under the strange influence of a Kirk Speech, the Companion relents, frees the men, and gives up its immortality to enter the body of the dying Woman of the Week to become Cochrane's physical lover.