This is a crossover bonus mini-series featuring both Ryujin and Akiko.
While the series is primarily focused on the characters of Ryujin and Akiko,many other heroes(Azriel,Haze) and even original characters may appear in the chapters.
This manga is different from all my others so far in that:
1)The pages shall be drawn horizontally
2)That series is wacky,next to non-serious and full of fanservice
3)Despite this,that series is canon.
4)They both also happen a little before the plots of RnS and KunoiEra
Hope you shall like