The Companion

The Companion was a female energy-based life form. She saved Zefram Cochrane's life when he would have died in space, and rejuvenated his body, keeping him alive far beyond his natural lifespan. She fell in love with him. 

When the crew of the USS Enterprise (NCC-1701) found the two of them on the planet, they tried to take Cochrane away from her. She resisted this, and attempted to keep the Enterprise landing party there as well. Captain Kirk and Spock attempted to kill her, but failed. Eventually, they convinced her that Cochrane would never love her, as she lacked a physical body. So, she merged with a dying member of the landing party, a human woman. Cochrane willingly remained on the planet to marry her. 

She died at the same time as Cochrane, a century later, aboard the USS Enterprise (NCC-1701-D). Captain Jean-Luc Picard agreed to position the ship so they could see the starfield they were familiar with.