Astarte is the goddess of everything ever and you will love her always even if you don't.
She will fight alongside you against the final boss.
You shouldn't let her die, or get low on health. Ever.
She also dies if you don't protect her constantly every chance you get.
You really don't want her to die because if she dies you lose everything.
She cannot be resurrected.
The fight will not continue without her, it will end the second she dies.