User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-172.56.4.215-20150315155335/@comment-24098614-20150315230732

It is only right that he called Issei a Hero because I remember when he tried to use the Will of God of his True Longinus it refused to destroy Issei also being due to his dreams, and his abandonment of his Humanity to win because Issei stayed true to what he was, and this was the reason the True Longinus accepted him for what he was.

Issei from the start has been a Hero because he has protected both the Underworld, and the Human world because it is true that he wants his Harem and to be with Rias but he doesn't want to see innocent people get hurt or killed because he is perverted but his heart is in the right place all the time.

He already has a quality of a Hero one is not never abandon your friends, and the second one is to protect the innocent from harm, and the third is to stay true to your ideals and goals never hesitating in what you want.