User blog comment:B214/Question and Answers 4/@comment-4547267-20141128000402/@comment-24098614-20141128085240

You can try to tone his perversion but is impossible to me at this point in the story to change him into something that he will never be because he is way too deep in corruption that is like trying to take away the nature of a Devil to have strong desires and their ambition and greed, which are traits that are impossible to remove from a Devil.

I don't think even a Exorcism can take away this trait from Issei even with a skilled person doing this that old man corrupted him way too much it can be toned down at a lower level but his perversion can be brought back to its full strength with the right amount of stimulation. This is my opinion in how hopeless it is to take away his perverted trait away from him I suggest that leave him as he is right now.