I'm just going to say this, but obviously, you have seen my rant with OCs and so forth, so I decided to do a different take.
First off, Issei is not the Sekiryuutei, he won't be till later.
Because of this, he will not be joining the supernatural world, but he will interact with beings, such as his childhood friend, Irina, and Xenovia, when it gets to that arc.
This story is more of Issei, proving that he is not just a pervert and going to expand on his other interests. Will he have perverted moments, sure, but he doesn't want that. This is more to build up his character as he tries to evade that stigma of being labelled a pervert, not just by the school, but also the supernatural. He is changing his dreams as he matures without the supernatural world until it brings him in, or rather till Ddraig brings him.
The Sekiryuutei will be an OC, but the focus is on Issei, you will see the other DxD characters, but it is on Issei.
It's as the story title goes, dude just needs a fighting chance to prove himself, that he's not just a pervert that you see in a lot of stories and with some adult guidance, maybe you'll see a different side of Issei, I have posted Issei's talents on my wall, so they will be featured here and the perversion will be toned down after an incident. That is all I have to say, thank you.