Explanations from a helper

"Ah, an expected reaction. I can assist you." A voice rang in Y/N's head as time seemed to slow.

"What?..."

"I'm a birthday fairy. I'm the one who granted your wish."

"What wish?"

"The wish for your perfect life. I sent you to another world where you can live at your best." The ethereal voice chirped.

"Do you do this often?"

"Oh, no, you're a special case. See, you've never used your birthday wish, not even once! That level of magic buildup had to go somewhere. Imagine our relief when you made a wish like you did last night."

"I didn't wish for this!"

"You wished for, and I quote 'I wish that from this day forward, I live the best life possible.' This is the best life possible for you in the known multiverse."

Biting back a flummoxed response, Y/N's rational brain finally took control back. He realized that shouting at the (not?) imaginary voice in his head that seemed to be capable of slowing time wasn't going to help matters. "Okay. Hopefully you can bring me up to speed?"

"Certainly. In this universe, you are the youngest child of Venelana Gremory."

"Like, from DxD? That would make Rias my sister..."

"Yes to both. They both love you very much."

"Okay...Am I still 18?"

"Of course. I can't manipulate age, that goes against the whole concept of birthdays."

"Okay...Anything else I should know?"

"Not right now. Call me in your mind if you have any questions are just want to chat. It gets lonely here at HQ..." The voice ended the conversation, and time resumed to normal speed.

"Sorry mom, sis. I did have a really weird dream last night, but I'm fine."

"Oh, well if you're sure. Hurry up and get downstairs for breakfast or you'll be late for school." Venelana walked out of the room, giving Y/N a great view of her backside. Rias followed, giving a similar show with the natural sway of her hips.

"Okay, now to get..." Going to the closet, Y/N noticed it was empty. "What!?"

A familiar feeling washed over him, invertedly slowing time. "Your family are naturists. They only wear clothes when they absolutely have to, which in this world, isn't much. Since you're 18, that includes you now."

"What!?"

"You didn't think Rias and Venelana both being nude when you met them was a coincidence, did you?"

"What about school?!"

"Your parents got you an exemption months ago. The only thing that needed to happen was for you to be of age."

Taking a breath, Y/N calmed down. "Okay...I guess naked mom and sis aren't so bad."

Knowing he didn't need to dress, Y/N exited his room and wondered what else this world had to offer.