Aeris?

The stars have been stolen (Meteor)
The the nightmares will come (Sephiroth/Jenova)
I was powers devotion (Aeris prays for Holy)
Now Gods maid of one (Aeris - God's Maid, one - last Ancient)
Swallow the tonic (I dunno)
Hold tight not to fear (Aeris pulls Cloud back into reality after Sephiroth fight)
For one day they shall thank me, (Thank you, Aeris!)
And I shall travel up another year (Traveling the lifestream?)

edit: Oooh, I like Ash's answer, too.