High School DxD, Vol. 5 (light novel)

A VACATION IN THE UNDERWORLD!School's out for the summer, and Issei's already dreaming up fantasies of hitting on hotties at the beach. His master, Rias Gremory, has other plans for him and the rest of the Occult Research Club, however—a season-long training stint in Hell. As if that wasn't disappointing enough, Issei's instructor is a terrifying dragon that tried to kill him! It may sound like a brutal lesson plan, but he'll need it because a few of Vali's allies decide to crash an underworld party and stir up trouble. What's more, one of them bears a striking resemblance to Koneko...