Please Fail Safely and Deadly

Brandon, the protagonist, is a highly-trained special agent. Astrith, the other protagonist, doesn't do anything other than... not dying.It is a law of this universe that she cannot die. She doesn't dodge bullets. Bullets dodge her.A nuclear missile is a different story though. To save the world (except our immortal protagonist) from being obliterated, and to retrieve a mysterious stolen casket, t…