In what must be a dream come true for Sayuki, Keiki takes her on a date to a theme park as his pet, and she must call him “master” and follow his commands. Of course, Keiki’s ulterior motive is to confirm whether Sayuki is Cinderella. The proof he seeks is whether she’s wearing the stolen underwear from his room originally gifted to him by Cinderella.
Sayuki wears a skirt for their date, but it’s long enough that none of the thrilling rides they take cause it to flip up in his field of vision. Like his date with Yuika earlier in the show, it actually goes quite well, aside from the investigation. After “being mean” with scary rides, he lets her decide to go on the Ferris wheel, and then he praises her shodou at an exhibition.
But while the Ferris wheel was definitely the place to do it, Keiki waits until the last second, at the end of the date, to order Sayuki to show him what’s under her skirt. As his dutiful pet she obeys, despite the embarrassment … because she’s not wearing any underwear at all. She went commando for the whole date.
In addition to neither confirming or denying whether Sayuki is Cinderella, Keiki finds that things are awkward at school the next day, with Sayuki not acting like herself. Yuika notices the change too, and the avowed sadist tries to make things worse by flirting with Keiki right in front of her.
Throughout Yuika’s initial attempts, Sayuki continues to write “patience” over and over on a parchment, until she runs off onto the table. By the time Yuika jumps into Keiki’s arms and they’re tumbling to the floor just as Mao enters, Sayuki has seemingly “hit her limit” and flees without a word.
When Keiki comes to them for advice , Ootori and Shouma can’t really say what’s bothering Sayuki, but the fact that it’s bothering Keiki means he must truly care about her, otherwise he wouldn’t be so worried about how she’s feeling.
The next day Keiki confronts Sayuki, and learns that she was acting the way she was because she thought he was playing “hard to get” after not doing anything to her after she lifted her skirt for him. She was hoping for a “healthy spanking,” and took getting nothing instead (and dealing with the rising anticipation) as a form of discipline in and of itself.
Keiki decides to ask her straight up if she stole the underwear, and she owns up to it. The only problem is, she didn’t steal Cinderella’s underwear, she stole—and is presently wearing—Keiki’s boxers. If only he’d asked the right question: Did she write him a love letter with the blue underwear?
So the mystery continues, because Keiki is an awful investigator. It’s raining after school, so he walks the umbrella-less Yuika home, but gets cold and wet so she invites him in. Rather than simply ask her directly, he rifles through her underwear drawer while she’s making tea, and gets an incriminating picture of himself snapped for his trouble.
That photo all but ensures he’ll be Yuika’s slave for as long as she has it in her possession. Why he thought the risk of getting caught in her underwear drawer was lower than the risk of alienating her for asking if she’s Cinderella, I have no idea. Keiki just can’t get out of his own way here!