Variante

[From SnowKiss]:

Aiko should be dead, but when she awakes in a secret hospital, she has no recollection of what happened that night until a monster showed up and her memories flood back. Her arm changed into a powerful weapon similar to the monster, a chimera arm.

The secret organization Atheos asked her to help them fight off the chimeras. An accident happened that led Aiko to understand how much she wants to live, and decided to join Atheos, in order to find her place.