Our heroes, obviously!

Sora quickly bursts back into the bedroom he shares with his step-sister.

"Yes, I saw it." Shiro confirms quietly, without even waiting for her step-brother to ask the question.

"We need to test these new rules. Quickly, and extensively." Sora says softly, but firmly.

"I know. And you could at least wish me a happy birthday first!" Shiro replies, pouting.

"Sorry."

Sora rubs the back of his head and gives a wry grin. He and his step-sister are very close, and he already knows that nothing this world can throw at them will shake their bond. Still, this might be awkward.

As usual, Shiro is already in business-mode.

"I challenge you to a game where the winner gains one gold piece from the loser." She states.

"Understood. I propose we play a game where after the count of three, we each turn around and show each other our buttocks. We can either do this while holding neither of our buttcheeks open, or pulling one of our buttcheeks open, or pulling both of our buttcheeks open, fully exposing our asshole. It'll work like rock paper scissors, where 1 buttcheek beats 0 buttcheeks, 2 buttcheeks beat 1 buttcheek, and 0 buttcheeks beats 2 buttcheeks. We will call the game 'buttcheek battle' for easy reference in the future." Sora quickly explains.

Shiro wrinkles her nose. "This is purely scientific, right? You're not going to get a kick out of this?"

Sora quickly shakes his head. "I'm going to find this as awkward as you."

"Okay, then I agree."

They both say "Aschente" at the same time, which means "I swear by the pledges".

And then, to their surprise, a parchment appears in Shiro's hand.

"But we made bets of equal value..." Sora says thoughtfully.

"They are bets of equal objective value, but clearly you care about gold pieces a tiny bit more than I do. Therefore they have more value to you, so this forfeit has been created to balance out the total value of the bet." Shiro concludes. "It says that if I lose, you have to put the tip of one finger inside my butt."

"What?! I'm included in the forfeit even if I win?!" Sora exclaims. "But I don't want to put my finger in your butt!"

"Hmm, assuming that's true, the forfeit can still make sense, if I don't want your finger in my butt even more than you don't want to put it in my butt." Shiro reasons.

The birthday girl sighs. "We still need to test several things, and the most straightforward way is for me to lose this bet. You do 0 cheeks, and I'll do 2. I'm going to be showing you my butthole anyway, after all."

Sora nods thoughtfully. "Agreed. On the count of three, then. One, two, three!"

They both turn around and expose their butts to each other.