A decent chunk of the premise for this started with me asking the question "How can I get Issei to tell Vali Lucifer 'Look, I know Ddraig and Albion have got this whole kiss-kill-kiss thing going on... But I'm a dude. You're a dude. If that's your thing, that's cool... But it's not mine. So maybe, I dunno, chill out? I'm pretty sure my liver will give out in a few years, then you can go lust angrily after the next one. Fifty-fifty odds that it's a chick, am I right?"
Semi-competent would be the key words, here. He's figured out a way to access magic, sure, but it has some fun and some not-so-fun drawbacks.