She recognizes who truly are the two Source Hunters right after she saw them, and as Source Hunters gather Star Stones and awakens as the Guardians, Astarte fills with joy and gets hope to finish the eternal battle against the Void Dragon which she mistakenly started by opening the Godbox. The Trife allured her into opening the Box. She was dwelling in the First Garden where the Source was born.
At the final battle against the Void Dragon, Astarte casts Mass Bless spell which increases nearby allies' chance to hit by 30%. This comes in very handy because Void Dragon's chance to evade is extremely high. She also casts healing spell too.
If she dies, the game automatically ends and you fail to win the battle against the Void Dragon. So you must keep her safe, by sending her far away using Feather Drop spell or by healing her more than the damage she takes.