The Companion was a nonhumanoid lifeform native to a planetoid in the Gamma Canaris region. Composed chiefly of ionized hydrogen and electricity, it appeared as a gaseous, vaguely sparkling cloud of yellow, orange, and red.
Encountered first by Zefram Cochrane shortly after 2119, it discovered him, old and dying, and brought him to its home, rejuvenating him and sustaining him for the next 150 years. The next Humans it encountered were a party of four: three USS Enterprise officers and a Federation commissioner, Nancy Hedford, in 2267. Responding to Cochrane's loneliness, it drew their shuttlecraft to its home planetoid so that they might be companions for Cochrane. This was a problem for Hedford, who was then suffering from Sakuro's Disease.
Zefram Cochrane could communicate with the Companion, in a limited and non-verbal way. According to his account, he "just sort of cleared" his mind, and it came. Despite the non-verbal nature of the communication, Cochrane claimed he was usually able to get his idea across. The Companion was able to answer at least simple questions in the same fashion – when Cochrane asked whether it could cure Nancy Hedford's illness, it was able to tell him that it could not.
Kirk tried a number of strategems in an attempt to regain his freedom, without much success. He first tried to disable it with an electronic scrambler. This device evidently caused the entity some discomfort or pain, as it reacted immediately, attacking and destroying the device, and nearly killing Kirk and Spock. (During this attack, the Companion changed from a soft golden color to an angry reddish hue, suggestion that its external appearance might offer some clue to its emotional state.)
Kirk next instructed Spock to modify the universal translator so that communication might be possible. Although it could now understand the Humans, attempts to persuade it to release them met with resistance. It wanted Cochrane to remain alive, and it believed he needed other Humans for that.
Finally, after realizing that it was in love with Cochrane, Kirk played on its inability to ever really experience true love with Cochrane, because they would always be too different. To overcome this, the Companion merged with Nancy Hedford, who was moments from death, creating a joined entity. In this merged form, it gave up its immortality and its powers, choosing to live out the remainder of a Human lifespan with Cochrane. (TOS: "Metamorphosis")
The Companion demonstrated a panoply of abilities, including:
- Traveling through space at warp speed
- Overloading energy producing and using systems
- Gross manipulation of objects, perhaps by some process analogous to telekinesis
- Produce material objects, including food and water
- Disperse or teleport itself
- Attack people and objects with electricity
- Rejuvenate humanoid life, and inhibit the aging process
- Merging with a humanoid, creating a joined entity. In doing this, the creature sacrificed its immortality and other powers.