Prologue

You have one friend.

A lot of people would consider that a few too many, but when it's a girl as troublesome as her – one is enough. Her name is Mitsuru, and you've been friends since your early childhood.

She's is a short girl with a bowl cut and big, round glasses. She is rarely seen without her white lab coat, which she often brought to your school without permission. Years of lectures from various teachers did not cause her to waver in her cause. She's no poser, the girl is a certified genius. When you were in fifth grade she was doing advanced calculus and reading university papers about astrophysics. By comparison you're as dumb as a brick.

You formed an unconventional friendship – although more often it feels like you're just being the straight man to whatever bizarre scheme or plan she's concocted while you weren't looking. She has no common sense. You feel a small amount of responsibility to make sure such a gifted mind doesn't die after failing to look both ways before crossing the street. You stuck by her side through both of your respective educations, and now live on a subsidized income earned from her various patents and discoveries as her only employee.

It started out like any other day, you didn't have anything to do and she needed a willing test subject slash lab assistant to help out at her lab. So after receiving an urgent text message from her you headed over. The lab was located in a large courtyard where several other businesses made their home, mostly garages and other storage facilities that could use the space. The building used to be a garage itself, used for fixing cars and the like. Mitsuru had bought the place at a bargain price and turned it into a makeshift laboratory.

She could have easily found a better location, but Mitsuru refused to be chained down by 'the man' or the 'intellectual establishment,' her desire to work unconstrained by ethics or direction culminated in her creating her own laboratory and working from it. You duck under the still open garage door and walk into the main floor, where the previous equipment for lifting and fixing cars still remained.

Mitsuru is in her usual place, stood at the back of the lab near her workbench. The back wall has been covered from top to bottom in whiteboards found online and covered with hundreds of incomprehensible squiggles and diagrams. Only she knows the real meaning of them. You note with some humour that the very top of the boards remain untouched, as she's not gifted in the stature department as is her brain. She hasn't grown an inch in ten years.

She turns to face you and smiles, "Good morning Assistant one, I have some wonderful news!"

Your heart sinks at the words 'wonderful news,' "What is it? Did the feds finally catch on to you?"

"Pah, those two-bit savages couldn't tie their own shoes properly, never mind discover my location… That's not what I mean, I've finally completed my latest and greatest invention yet!" With no sense of theatricality, she dashes off to one side and wheels over a small cart once used to carry various tools used by the garage. On top of it is a strange gun like device, white in colour, and a single playing card placed next to it.

"I call it the Sledgehammer, patent pending."

"Why is it called the Sledgehammer? And isn't this kind of illegal?"

"It's not illegal, assistant. It doesn't fire ammunition! It's totally harmless!"

"If you say so."

"As for why it's called the Sledgehammer – this device is capable of a damn sight more than merely firing a projectile at high speed. It uses highly energized anti-matter to punch a hole in the fabric of our reality, it's really rather simple."

"…Mitsuru, that sounds even more illegal than it just being a gun! Not to mention a million times more dangerous!"

Mitsuru waves away your concerns, "When have my inventions ever caused you harm?"

"Do you really want me to list every single one?"

Mitsuru is quick to change the subject, "On this I can assure you, the Sledgehammer, name pending further development, is entirely safe to use."

You hesitantly approach the cart and lean in for a closer look. The device is strange, it's shaped like a rifle if it ate a little too much. There's a glass chamber embedded in the middle, and a large handle hanging off one side for you to hold on to. It looks like the front assembly can twist upside down.

Next to it is the card, which at first didn't attract your interest, but now that you're closer…

A card with a yellow border and red background, along the bottom in bold white text is the name "RIAS." The card face is adorned with a familiar image. Rias Gremory. From High School DxD, in all of her animated glory.

You turn to Mitsuru and hold up the card, "What's this? Did you just drop a trading card in here for dramatic effect?"

"It's a little more than a trading card," Mitsuru pouts. In typical Mitsuru fashion she then elects to not explain the significance of said card, snatching it from between your grubby fingers and holding it up to the light. You have to scratch your head. Mitsuru didn't like that show one bit when you both sat at your old laptop and watched it together while hiding from your parents. Too much fanservice for boys you reckoned. Young you loved it, you felt like such a rebel for watching something with boobs in it that you could ignore the other, less admirable elements.

"So why did you make a gun that can tear a hole in reality anyway?"

"There's a simple explanation for that, you see.."

Before Mitsuru can explain her intentions, the entire lab shakes. At first you think it's an earthquake, but it settles down soon after. Mitsuru checks her watched, "Ah crap, they arrived earlier than anticipated."

"Who?" Mitsuru shoves the gun and the card into your arms and runs as fast as her little legs can carry her through the front door. "Seriously, answer my questions for once!" You duck under the doorway and skid to a halt next to your friend, who is staring down what can best be described as a golem. A shifting mass of rock and stone forced into the shape of a hulking creature. Glowing purple eyes stare down at you, seemingly focused on the gun currently clutched between your shaking arms.

"What the hell is that?"

"No time to explain, we need to use the Sledgehammer now! Put the card in there!"

There's an open slot for the card that Mitsuru gave you. You flip it over to find a list of statistics including her combat strength, and more strangely the size of her bust. There's little time to consider the potential implications of using this thing. Mitsuru may lie by omission now and again, but she's never done anything bad to you on purpose. You slide the strange card into the gun's chamber and push the lever forward, locking the chamber shut.

"RIAS GREMORY!"

The strange device declares in a robotic voice. "Now twist the front around using the handle!" You follow the rest of Mitsuru's instructions and twist the handle and the barrel assembly around to the left.

"SHATTERING!"

Mitsuru points at the monster, "Fire!"

You wince and pull the trigger. The gun's recoil is so powerful that you're knocked flat on your ass. A large ball of purple energy fires from the muzzle of the cannon and hits an invisible target in front of you. Reality shatters like a broken mirror before your eyes as the shot breaks through the dimensional barrier. The scattered shards reassemble themselves into the form of a woman. Colour fills out the mysterious silhouette from the bottom of her feet to the crimson hair atop her head. The gaping hole in the universe floods back inwards like water, leaving behind the titular demoness.

For a moment she is confused by her new surroundings, but as the monster swings at her, instinct takes over and she leaps into action. She flies over the wild swing with the grace of a ballet dancer, landing firmly on her feet. With a pretty scowl on her face, she holds out her right hand and fires a barrage of red lightning at the enemy beast. Wind blows through your hair as it explodes into a million little pieces, leaving nothing behind but a smoking crater and a glowing purple gem.

The entire thing was over in a moment.

You try to blink the stars out of your eyes, what on earth was that? The girl in front of you blasted that beastie to smithereens like it was nothing but an ant! Mitsuru hurries past you and slides into the crater, she pulls out a small glass tube and places the stone inside of it, before slipping it into the pocket of her lab coat.

You have a lot of questions and no answers, you tug on Mitsuru's sleeve as she walks by, "What the hell is going on? What is this thing? And why is Rias standing in front of me right now?!"

You want to think that this is merely a trick of the eye being played on you, but the sensations were very much real. Rias Gremory is every bit as beautiful as implied by her appearances in the anime and light novel series, now she stands in front of you – transported into the real world, wearing the same school uniform and wielding the same destructive powers.

Mitsuru adjusts her glasses, "I may have discovered some interesting alternate dimensions, and perhaps in a few of those we're the… main characters in a popular fictional anime series."

"You're messing with me."

"She's standing right there isn't she?" she says, motioning to the starstruck she-devil.

"Is she messing with me?" you ask.

Rias shakes her head, "Ah. As remiss as I am to admit my brother used to enjoy it very much, it rubbed off on me. Soon I found myself independently infatuated with your adventures. To be summoned by you just like in the anime is amazing!"

"And furthermore…" Mitsuru continues. Unfortunately, she isn't able to explain any further before the busty beauty pulls you into a bone-crushing hug.

"Oh, you're just as handsome as you are in the anime!" she gushes.

While normally you'd enjoy the feeling of a pair of DD breasts against your face - she is applying a worrying amount of force to your neck, "Mitsuru, help!"

"…They also happen to be rather big fans of you in particular, thanks to the way that I calibrated the cards."

Rias finally releases you, but sticks herself to your left arm like a doting girlfriend, "What does that mean?"

"What do they call it again? Hah! I remember, you're their husbando."

Rias is quick to speak over the diminutive scientist, "Oh! I have an idea for how you can repay me! Take me out on a date. You don't mind, do you?"

"It's a rather simple equation. I use my cross-dimensional sledgehammer to bring forth helpful girls, who all happen to be in love with a fictionalized version of yourself. We claim the cross-dimensional material dropped by these creatures and use it to create even more cards and expand your harem. An elegant solution, if I do say so myself."

Your mind is being overloaded with information right now. Rias wants to date you, you're a fictional character in another universe, and this crazy girl wants you to start a harem of badass warrior women to fight evil?

"I know I've said this so many times before, but I think you've really lost it this time."

For some reason the utterance excites Rias to an incredible degree, "He really said it!" she cries out with the energy of someone having just heard their favourite catchphrase in real life. Mitsuru sighs and cleans off her glasses with a small cloth.

"To keep things simple, an army on interdimensional invaders is here to conquer the Earth. You and me are the only people who can stop it."

"The Untethered," Rias nods.

"Well, I'm not sure if they're really called that, I suppose if we follow the hypothesis that the TV show you speak of is entirely accurate it could be reasonable to assume."

You jaw drops as the two exposit back and forth on this series of developments, "How is any of this simple?! And is Rias just stuck here forever now?"

Mitsuru taps her temple, "No, no. Of course not. You merely reverse the steps taken before, and Rias will be whisked away to her original dimension. However, I get the feeling that you may wish to spend some time with your new harem member."

You object, "Who says it's a harem?"

"I wouldn't mind," Rias smiles, "You're really cute."

Your face flushes, you look away and try to not let them see you. "See! He's such a cute boy! How have you not snatched him up already?" she asks Mitsuru, who is too distracted admiring her newly acquired gem to notice.

"Hm, oh yes, have fun you two!"

Mitsuru hurries back into the lab, leaving you and Rias behind.