His mom!

"Shit, that's Mom... what do we do!?" he hissed to Rias as Issei's mother knocked on the door, wanting to know when the hell he had gotten home.

"Just wish for her to not notice anything off about this, Master!" Rias giggled at him, and the knocking stopped.

"...Issei... do you actually have a girl in there?" Miki asked from the other side of the door, stunned.

"She makes it sound like some kind of international incident!" Issei thought. Honestly, he would be a bit more indignant about it, but given his own perverted tendencies, he had to admit, it was genuinely surprising from an outside perspective. He had barely believed that he had gotten a girlfriend before... then again, given what Rias had said, Yuma had only been with him to kill him. "Yes, I wish she wouldn't find anything about all this odd!" he quickly hissed to Rias. "Dad either!"

Rias giggled and snapped her fingers. There was a pause. "...don't take too long Issei, you still have school today!" Miki then said from the other side of the door, like she was just giving him a standard lecture, before heading back downstairs.

Issei breathed out a sigh of relief. "...she's not wrong, we do still need to go to school," he noted. He then looked at Rias's tail, and paused. "...actually, how are you going to get to school like that?"

Rias smirked at him. "I only need your permission to have legs, Master," she chimed. "Otherwise, I'm stuck wherever the lamp is. And before you ask, no, I can't move it. It basically has infinite weight for its genie. The tail is like both a leash and a chastity belt built right into Genie biology..." she crooned, clearly aroused by the prospect.

"Right. Then reform your legs whenever you need to, consider this long term permission," he said after a moment.

"Yes, Master!" she nodded, her tail detaching from the spout of the lamp, and splitting before shifting back into her legs, complete with the lower half of her harem outfit. "Let's get ready for school then, shall we?" she smirked as she took off her veil and went to where she had discarded her school uniform.