Chrome Shelled Regios: Missing Mail

In this school fantasy story set in the distant future, creatures contaminated by pollution have overtaken the land, forcing humanity to live in isolated mobile cities called Regios. One such city named Zuellni is a self-contained school in which freshman Layfon Alseif has just enrolled. Layfon reluctantly joins the 17th Martial Arts Studies Unit led by the third-year student Nina Antalk and embarks on a series of adventures.
This version recounts a slightly different version of Layfon's story. Here, Felli is the main character. Same world settings as the original novel.