Last Boss no Mukougawa

Last Boss no Mukougawa (ラスボスの向こう側, lit. "The other side beyond the last boss") is a light novel series by Noir Amane with illustrations by Gochou.

Plot
A 16-year old boy with a love for observation dies in an accident. God appears before him, informing him he'd be the perfect candidate for a quite unorthodox reincarnation: an evil god named Ashtal. As said evil god, it's up to Ashtal to serve as a 'hidden boss'. Just like in an RPG, Ashtal will only appear after the 'final boss' is slain in battle. This gives Ashtal the opportunity to observe the world as much as he wants without anyone interfering.

Ashtal waits for over a 1000 years, yet no band of heroes arrive to kill him. However, finally, after longing for some heroes to show up at his doorstep for 1005 years, a group of heroes, led by the second princess of the Britton Kingdom named Yufilia Plantagenet, barge in to slay the mythic "Evil God".

After a rather embarrassing encounter with this all-female group of heroes, Ashtal decides to quit his life of observation to attend school with the ones who came to slay him.