Unfortunately, there was an imminent threat that Issei's girlfriend was going to castrate him if he left her sight for any longer than thirty minutes, so under recommendation from Rias you decided to send him back to their world before that could happen. Before you left the scene, you spotted a red scale resting inside one of the craters. It must have been knocked off of Issei's armour during the battle. Not wanting the enemy to take anything away from the fight, you retrieved it and put it into your pocket.
With your first date with Asia wrapped up and the invaders repulsed once more, you decide to retire to the garage and see if Mitsuru had made any progress on her form change device. You can hear the whirring of her machinery as you approach the front door. She's hunched over a bench with plastic goggles wrapped firmly around her head. Sparks fly from beneath her.
"It appears that Mitsuru-san is busy..." Asia whispers.
The whirring stops, and Mitsuru places a small tool onto the bench beside her; "Actually. I just finished. Thanks to my own intervention completing the advanced translocator was easy enough that even a dullard could have done it. Though there is still one missing component."
You chuckle, "What? He didn't even tell you how to complete it?"
Mitsuru opened the top of the device and showed the internals to you. A large space had been left behind. It was entirely intentional. The arrangement of the parts that had been hinted to in the paper had made that space on purpose, and several diodes pointed inwards to connect to whatever that missing piece is. As you start at the shape, an idea starts to form in your mind.
You reach into your pocket and place Issei's scale down onto the work desk. What a freakish coincidence. It was almost like your son from the future already knew that this was going to happen. Mitsuru reaches over and holds the scale over the slot, her brow furrowing in irritation.
"Typical. I was getting worked up over nothing." Mitsuru didn't hesitate, sliding the scale into place and pinching each side with a small conductive clip to connect it to the system. The mechanism whirs to life for the first time. Mitsuru does a small jig in celebration; "My future genius knows no bounds."
"Isn't this kinda' cheating?"
"I still discovered it in the end!" she snaps back. Though this time loop does beg the question of who discovered this thing originally. Since Mitsuru is informing herself about how it works, it technically means that nobody has ever come up with this without outside intervention. This is both extremely confusing and not worth the effort to think about – so you discard the problem and just roll with it.
Mitsuru returns to the computer and checks the readings, "That scale contains an incredible amount of power. This is what my future self must have intended."
"So does that mean I can do this form change thing now?"
Mitsuru waves her hand in the air, "Give me a few hours to figure this out. I should be able to use the data from the scale and your previous battle to make something special for you."
"As long as it means I can deal with Vali myself..."
Rias shrugs, "Issei will never object to knocking him down a few pegs."
"Sure. But at what cost?"
"She isn't going to castrate him that badly," Rias smiles.
"I'm sure she's really happy that her boyfriend is in a club filled with nothing but beautiful women."
Rias smirks, "And a beautiful boy."
"Sure. Kiba too."
"I've never been called beautiful before," Asia whispers. Given that she has the face of an angel and pretty blonde hair – you aren't really sure that's the case. Were all of the people at her church blind or what? And while you'll never reveal this out loud, you've seen more than enough of her body naked to know it's no slouch in the curves department; unless you're comparing her to Rias and Akeno. That's an unrealistic standard to attain for most. They both have bodies that a perverted twelve-year-old would scribble down onto a scrap piece of paper for fun.
"I guess that means our date is over with, and Mitsuru finally has what she needs to finish her device so... what now?" you grunt.
Rias jumps at the chance to tease Asia a little; "You could take her down to the special 'chamber' and show her how much fun being your lover can be." Asia squeaks and covers her face all over again.
"I-I don't know anything about that."
"About what?" Rias prods.
"S-Sex!"
Asia shouts it so loud that if anyone were still present on the industrial lot, they would have all have heard it. She stomps her feet on the floor and runs to hide from Rias as she continues to hound her with more sing-song questions about making love and being your girlfriend.
"She can be very mean when she wants to be," Mitsuru comments glibly.
"Eh. I think she likes teasing the members of the club."
"But she was rather jealous in the original, was she not?"
"But you brought her from a world where devils have multiple spouses – of course she's not going to find it upsetting when she's been raised that way from birth. I think being positioned as the head wife is also going to keep her happy."
As you watch Mitsuru hammer out hundreds of lines of code at an inhuman speed, you finally ask something that's been on your mind for a long time now.
"And how do you feel about it? You don't find any of this stuff weird?"
She shrugs, "I was the one who came up with it – and we've both long been immersed in the non-traditional. Every indecisive otaku dreams of having a big old harem from time to time."
But what about your relationship with Mitsuru? It feels like everything she's doing is for your benefit alone. Sure, they help you fight off the aliens, but you're living the high life with a dozen devoted partners while she slaves away to make new weapons for you. You haven't done any of the fun stuff you usually do together for months now.
"Are you worried about our friendship?" she asks pointedly, "It's written all over your face."
"There's nothing wrong with that."
Mitsuru pauses for a moment and turns back to you, "I feel an elevated sense of responsibility to protect this planet. We are the only ones who can do this. The fun and games can wait for later."
"You haven't thought about summoning a few wish fulfilment picks of your own? We have some spare space rocks..."
There's a long pause as your suggestion hangs heavy in the air. Mitsuru blushes and returns to her work in a huff, "Not interested."
"Really? Not a single anime guy you'd like to make into your boyfriend?"
"Not interested," she repeats sternly. It's the kind of reaction that you only ever elicit when you're pushing a button that she doesn't want pushed. She really means it. You can't inquire any further as Rias bursts out from the side office, dragging Asia with her by the collar.
"If you're going to be a member of the harem, you have to get used to that kind of talk! A good wife should help take care of the husband's sexual needs. It's perfectly natural."
"B-But the church never talks about things like this!"
"Then we will simply have to provide you with an effective and detailed sexual education!"
Asia is still trying to wriggle free even as Rias hugs her in mock compassion, "Please, president!"
You step in to break things up, "Come on Rias. I don't want to pressure her into anything she isn't ready for."
Rias pouts and allows Asia to hide behind you, "Very well."
"Did you have fun Asia?" you ask.
Asia smiles, "Y-Yes. Thank you very much for bringing me." She wraps her arms around your waist and hugs you close from behind. She then backs away and bows politely to you, "I would love to do it again."
"That's the spirit," Rias grins, "As long as you endeavour to become a good wife, you will make progress."
"A good wife... I need to do more research to find out what a good wife does."
Rias approaches you and pulls you down into a kiss, "It looks like it's time for us to go. Don't be a stranger, hubby!"
"Of course. I'll pencil something else in again soon."
You say one last goodbye to Asia and send both girls home by resetting the Sledgehammer. Issei is going to have a big story to tell all of his friends about, if they're willing to believe that he was summoned by a 'fictional' character. You mull around the lab for a little longer – coming across a set of plans on one of the tables for renovations to the industrial estate.
"I just bought a big house out in the sticks. Are you sure you don't want to build a secret lab under it?"
"It's easier to do here," Mitsuru explains, "These buildings are already tooled and equipped for the kind of industrial work I'm planning to do. If I moved to your new house, we would need to completely change the surrounding infrastructure to handle power consumption, for one example."
Some of the older, more rundown buildings are going to be demolished and replaced with new ones. The disparate pieces of the industrial estate are going to be connected together into one larger building. The underground sections are also being enlarged with some help from Lala's warp space rooms – though those plans are specifically marked with big red lettering to prevent them from being handed in on accident.
There is also a plethora of features designed to protect the lab from attack or investigation. Machines that won't be used, buildings that are just there for decoration, full security camera footage, motion sensors and alarms, and doors that are impenetrable from the outside. The entire thing is a convincing façade for a local research and development company. The detail involved means that Venelana's fingerprints are all over it.
These expanded spaces would allow Mitsuru to purchase and use much more advanced testing machinery. Right now, she has to make do with whatever she can fit into the garage, which means large devices are mostly out of the question. If she can do this much with such a sub-optimal workspace you shudder to think of what she will create with the proper equipment.
Speaking of renovations, you need to follow up with Rias and Venelana about getting the mansion fixed up so you can use it properly. No doubt the pair have already drawn up detailed floor plans and picked out all of the custom-built furniture they want placed inside. Maybe you can summon a battle maid to help keep the place tidy, because it's way too big for one person to handle.
You can worry about that later. You need to start packing what few notable possessions you have into some boxes and get ready to move in.