Start with giving him a blowjob!

Issei considered it for several moments, before he settled on something every self respecting pervert wanted. "I think we can go with a blowjob, if you wouldn't mind giving me one?" he asked her, and she grinned.

"Master, I would not only not mind, but I love the idea! I have a very strong sense of compersion towards you, after all..." she purred, sliding off of his lap, and stroking his cock as she met his eyes with adoration in her own.

"...compersion?" Issei asked her curiously, moaning as her gentle grip slid back and forth on his manhood, and she giggled.

"Compersion is a form of empathetic emotion, where you feel fulfillment from the happiness of others as if it was your own, even if you don't directly benefit from what makes that other person happy yourself," she explained, using her free hand to pull up her veil, and leaned in, extending her tongue to lick around the head of his penis.

"Oh... I see..." he panted. She let her veil drop again, letting it drape over his shaft, before she stopped licking it for a moment.

"Just so you know, Master... you're free to talk dirty to me if you wish to do so..." she crooned at him. She then engulfed his shaft and began to bob her head on it, suckling like a starving calf on its mother's teat.

He gasped at both that, and her words. He bucked his hips a bit as he clenched the sheets. "You mean... say things like calling you a slut?" he panted out.

"Mmmhhhmmm!" she replied in a drawn out manner, sending pleasurable vibrations through his shaft, making him moan. She didn't even look just fine with the prospect, she was practically begging him to talk like that to her during sex, she was communicating it clearly with her eyes.

As she continued to thrust her head back and forth, he panted, hesitating slightly. It seemed so... demeaning... but was it really? Rias was definitely acting like a slut, and wasn't the least bit ashamed of it. In fact, she reveled in it. That made him decide to give it an honest try. "Okay then, um... take it all, like the good slut you are..." he stated hesitantly.

She giggled in amusement around his cock as she continued to suck it hungrily. Despite his hesitation, she was pleased with it. He'd get better at talking dirty to her with practice. For now, she was focused on drawing out his pleasure to the maximum degree. "That's it..." he panted as his cock throbbed, his pleasure building to its apex. "Have your fill, slut!" he grunted, his confidence with dominant talk growing a bit as his seed gushed into her mouth and down her throat. She didn't hesitate to swallow it all down, before pulling off the moment his shaft was spent, and she gave his manhood a playful lick from root to tip.

"Mmm... so good, Master~!" she cooed as he wheezed, his dick softening.