Issei was an honest guy—at least, when it came to his desires. Either way, he didn't want to cheat on something as important as a relationship, so he couldn't let himself make the date go well, but what should he do, then?
Maybe he needs a dating coach, or someone else who can give him advice! That wouldn't be cheating, right? Right.
And if the book can warp reality, then his coach can look like anything he wants them to, right?
Or at the very least, he can make sure that they don't look like something he doesn't want them to look like!
He doesn't think he could stand getting advice from an ikemen for very long before he'd want to punch something.
So, probably a cute girl, then! But, well...he'd want to actually listen to her, so she couldn't have super prominent boobs, because he'd end up staring at them at least half the time.
And, she should also probably be invisible to other people, right? Like a ghost or something. It'd be awkward trying to explain her sudden presence to people. And she probably shouldn't be heard by other people, either.
(The fact that he could use the book to solve that problem had not crossed his mind. To be fair, he is tired, and it is late.)
Okay! It's decided then!
He wrote "I have a spirit guide that follows me around. She is a cute girl whose boobs won't distract me from listening to her advice. The advice she gives is amazing and is always helpful. She cannot be seen or heard by anyone but me."
Satisfied, he took his pencil off of the paper, and...
"So, you're my master, huh? Or would it be 'student', then?" He heard from behind him.
He turned around, and saw that his guide was...