His use of Rias, and his potential! They discuss it over sex!

"Well, for starters, just look at what you've already done with my powers to aid our training. We can recharge with an orgasm, and almost tripled our training time, thanks to your creativity," Rias told him as she lais back on the table, spreading her legs, presenting her pussy for him. The invitation couldn't be clearer if she had put up a neon sign.

He stood, pulling down his sweat pants, and let his erection spring free. "I suppose that's true... But anyone with your Lamp could have done that..." he told her as he stepped between her legs and rested her hands on her thighs. The tip of his cock probed at her lower gates.

She smirked, wriggling her hips enticingly. "Maybe, but only if they had thought of it. You have some good ingenuity with my powers," she assured him, before giving a blissful sigh as he thrust into her. "We'll all be able to get stronger than we would have otherwise because of this..." she panted out.

Groaning, he conceded the first point, and began to thrust in and out of her slowly. "Okay, so, you said that was just the first point... What's the next?"

She gripped the edge of the table, and wrapped her legs around his waist as he kept going. "You said you were the weakest of us..." she panted, her vaginal walls massaging and milking his length. "That's only half true. When you're not using your Boosted Gear, or just starting a battle, that is the case. But after enough Boosts, you can get strong enough to pose a threat to many enemies. So long as your body can handle the Boosts in question."

"Wait, there's a limit to how many times I can Boost?" Issei questioned, gradually increasing his speed, but still not going particularly fast. Both to help focus on the conversation, and to savor this round.

"Yes," Rias admitted. "Mmnh... Your body at its current level, can only handle so much of a power increase... You were able to put out so many against Raynare because your active sex life had helped your fitness a little, and because you hadn't promoted, which left you with less power to repeatedly double to begin with. But there was a reason you were tired enough to stop at just one round with Asia that night, even if your primary reason was to keep her first time special. You had strained your body probably more than you realized. A couple more Boosts would have made the extra fatigue noticable, and not easily chalked up to the fight you just had."

He grimaced, though he continued to thrust. He was now going at a fairly high speed, his manhood going back and forth into her and stimulating her most sensitive spots, making her croon. "I see, so that's another reason we're working on my baseline strength, so I can handle increasing my power more..."

She nodded, arching her back in a manner that offered her boobs to him. "Yes. And also so you can handle using the power of all eight Pawn pieces..."

He slid his hands from her thighs, up her hips and abdomen, and rested them on her breasts. "What do you mean?" he inquired, giving them a squeeze and playing with them, leaning down a little and gazing into her eyes.

She smiled, pushing her hips against him in a wordless request for more speed and ****, and he decided to oblige, picking up the pace in her. "Even without the Promotion, the Pawn pieces provide a small boost to your overall stats. But despite the cost of reviving you being all eight, due to the value of your Boosted Gear, you still weren't prepared for the full power of all eight. So I sealed off all but one of them for the time being. I'm hoping we can get you ready for at least three before this training trip is done. Maybe even four."

Issei nodded as he realized how much sense that made. She had mentioned he had all eight Pawns in him, but he had only had about as much power as he would expect from a single one... But with all eight, they should have increased his abilities to be nearly on par with the speed, strength and magic of a Queen. How had that never occured to him? He suddenly felt a lot more confident in his overall potential.

With that in mind, he began fucking her as hard and fast as he could. "Yes, fuck me, Master, fuck me hard!" she gasped, before the door opened, and Raynare stepped in, freezing at what she heard and saw. Issei almost froze himself, but decided to finish the sex before addressing that particular issue.