Well enough, though he has to prove it a bit.

Kazuma took out the Sex Note, and gave her the run down about its power, and how he already had a happy harem with the rest of his party. She was however, skeptical. "Yeah, pull the other one," she giggled.

"Not everyone is as much of a prankster as you are Fio," he deadpanned. "Also, to paraphrase Cremea a bit, you've seen the things I've been involved with as of late. Is this really so hard to believe?"

"I mean... Kind of," she replied. "You have to admit that this is a little bit far fetched, even for you..."

"...I suppose that's true," Kazuma chortled. "How about this then?" He opened the Note, and wrote aloud, "Whenever Kazuma Sato says, "The sexy joke is on you," to Fio, she will immediately have a long and intense orgasm."

After he closed the Note, she looked at him curiously. "...so... What, you say that, and I instantly cum?"

Kazuma smirked, and said, "The sexy joke is on you."

Immediately, Fio's eyes widened, and she fell back onto the bed. "Ah, oh, unh!" she gasped out as she spasmed and bucked her hips. "Holy shiiiiit!" she moaned, her eyes rolling up and her tongue lolling out. After a full minute of this, the orgasm subsided, and she limply relaxed on the bed. "...by Eris, that WAS awesome..." she grinned weakly. "I'll have some more of that Master..."

"Perhaps later, when you've recovered," he chuckled as he got into bed with her. "So then... You're fine with this I take it?"

"Oh hell yeah, if being your sex slave feels that good, and you don't mind me doing my own thing otherwise, I'm quite happy with this," she grinned, cuddling into him. They soon fell into a relaxed, tired slumber.