Bugged by My Tyrant

There is a system that hands out assignments in the past, and finishing one lets the person carrying it leave. Celeste comes from the present day with a single job, which is to turn the young emperor into a good ruler. She puts him on the throne herself and then arranges her own exit from the story, expecting that to be the end of her involvement. The alarm goes off instead. Grief over her turns him, and he becomes the most unhinged tyrant the records hold. She is sent back through three more lives to get him onto the right road again. One of those lives puts her in the palace as his adopted sister. The body is different and he knows her anyway, by the soul, and he will not be talked out of it. He takes hold of her wrist and tells her, "Celeste, this time you can't run." She works out eventually that what has been holding her in that world is something the tyrant asked the gods for.