He doesn't even know how to?? It will likely be something that he can do when he either has s0x or kills someone, I swear webnovel readers can't read for the life of them.
Hmm. At the moment I don't see an opportunity for mc to become stronger. As soon as Issei awakens the sacred mechanism, he will be stronger than mc. And Rias will sleep naked in Issei's bed.
