Satsuriku no Tenshi

A 13 year old girl named Ray awoke in the bottom of the building. She lost her memory and could not understand why she was there. It appeared there was a murderer, Zack, that was covered in a whole body bandage.
"I have a wish, please kill me"
"Help me get out of here with you, I will kill you then."
The strange ties between them will deepen their promises. Where are they supposed to be? For what purpose were they confined? What fate is to await them? A mission to escape from a sealed building begins...! The completely novelized version appears!

(Source: Amazon, translated)