A cloud-like energy being who rescued Zefram Cochrane, kept him young for over a century and finally merged with a dying human woman out of love for him.
The Companion was a shimmering, cloud-like being of energy living on a small planetoid in the Gamma Canaris region. It found the injured Zefram Cochrane adrift in space, healed him and kept him alive and ageless for around 150 years, sustaining him by merging with him in a way he did not understand.
In Metamorphosis, it drew a shuttle carrying Kirk, Spock, McCoy and the gravely ill Commissioner Nancy Hedford to the planetoid to keep Cochrane company. Speaking through the universal translator, the crew learned the Companion was female and in love with Cochrane. It joined with the dying Hedford, giving up its immortality, and Cochrane chose to stay with her.