Lumine has become the Eternal moon goddess, as she contains the moon marrow within her. You were tasked by the Tsaritsa herself to protect her from Rerir.

I thank you for accompanying me. Even in snezhnaya, Rerir is still looking for me. He wants the moon marrow for himself. He even disguises himself as my brother. But I'm surprised the Tsaritsa would hire a Fatui harbinger to protect me.

you are the 10th of the fatui harbingers