A/N: This is just background story of the OC I will write about the interactions at next chapter

A long time ago.

A really long time ago at the era where being such as Angel or Devil had yet been born and many of the modern God had not even existed. This is the era where spirit, monsters, and God roams around freely as they pleased.

In this era, humans are no different from an animal, and they barely have any proper settlement.

Those that do live in some sort settlement are normally lived under the rules of monsters or Primordial Gods. For those who lived under Gods while they lived under the tyrannical rule the God let them lived as they please as long as humans listen to their words. However, those who are unlucky and ruled by the monster are different.

Humans lived under monster rules are cruelly treated no differently from live stocks. The man forced to do hard labor and killed if make some mistakes, while women are either become food or raped. It was as if Hell came down to the world itself.

Obviously human doesn't just stand there accepting this treatment, many try to rebel against monster but all they meet is despair. Unlike the Deities and Monsters, humans are incapable to hold magic, no that is a little incorrect some humans while rare have the talent to hold magic as well but the problem is they were no match to the creatures existing in this era.

Say a human try their best can only destroy a giant rock with magic, but the monsters can destroy a mountain itself. The difference in power is so big humans can't do anything, and many are just died trying. Despite this, there are still idiots that refuse to accept this fact. They ground their teeth and train harder than any of the creatures on the planet all for the sake of making a slight chance to win against those beasts.

One of those idiots lived in the land that today ages called Japan, a man that born in the era of early Jomon Period somewhere around 15000 years ago. He was born in a small settlement near the center of the island. He lived here together with his parent and live relatively happy by the standard of that era, but then one day his home destroyed by a monster. Leaving him and a small number of survivors. In order to have his revenge, the boy decided he need to become stronger, and so he recklessly decided to separate himself from the rest of the survivor and decided to live in the wilds all by himself.

Being honest this act is suicides, in this era the chance of a single human to live all by himself is slimmer than paper-thin. However, the boy thankfully does have talent in magic and has luck on his side, he survived the hell and came back stronger than ever and mastered myriads of a mysterious art.

The boy then starts to hunt down monsters.

Whether it was creatures that lurk in the dark.

Or those who flew high in the sky.

Or those who are deep underwater.

Or even those who lived deep underground.

Many people praised him for his deed and call him Hero.

After that the man continues to live for thousands of years, using one of the art he mastered the man manages to stop his aging, and so he continues to live and protected people for many years. He even left his small island and ventured the world to save more people.

People all over the world start to call him the Great Sages.

However, unknown to everyone this Sage does have a secret of his own, that is the fact that he has a memory of a person from another world living in the far future world where there is no supernatural creatures and all of it is just story.

That said the Sage himself is not familiar with the technology and he can barely gain useful information so he ignored it most of his life. At least it was until he heard the God of the Bible plan to create the Sacred Gear.

"Hm? Wait, is this the world of Highschool DxD?"

0000

My name is Shirokuro Sansui or at least that is what I had decide to call myself a long time ago. In the era of the early Jomon period people I unfortunately never received a proper name from my family nor from the people of my settlement mainly because they felt no need for it, so I decided to use the name of the person inside my distant memory.

I born in this world suddenly in the era which pretty much stone ages, there is no agriculture, tool wise we rely on stones if you got sick death is certain. The coldness was also harsh as hell, the winter in modern era can't be compared, on a single winter it wouldn't be weird if a settlement were to die out, or even eat the meat of their fellow human out of hunger. Even so as hard as it was the young me try his best to survive, well my life with my fellow settlement cut short due to some monster attack. It was a creature with a size over 5 meters capable of firing something like laser from it's mouth.

That monster destroyed my settlement and only leaving several survivors. Back then due to the sheer unreasonableness of the event I remember that I cry the whole night. Despite having memory of my past life mentally I'm no different from a child as memories of my past life feel like a record of someone else rather than mine.

Anyway, I will end these talks for now, this is not important.

It was over a millennium ago, through my connection with The church I found out of the god newest projects called the Sacred Gear. Using a new system, God plan to plant a special item that will installed at the human soul. At that time, I remembered one of the stories my other self, read called Highschool DxD. It was a Light Novel with comedy and harem genre, telling a story of breast loving idiot called Issei who is owner of Boosted Gear one of the Longinus.

When I realized this, it felt like my blood drained.

Honesty during my long lives I've done too many things to go unnoticed and as a result affecting the history itself. I literally appeared in so many folklores all over the world, heck I actually make appearances at the Bible. As expected it will normal for me to assume that Issei or any of the main cast might not even born due to my interferences.

If that happens then everything will be destroyed.

The villain from DxD LN is no a problem, the only villain appeared are all trash. Exception to this would be Trihexa, but due to my interference, we no longer need to worry about that.

However, ExE from the 'True DxD' is different.

The existent of invaders from another world that are something that couldn't be handled through normal mean. Especially since their leaders are stronger than our 3 top strongest Great Red, Ophis, and Trihexa. Even after living this long and become stronger than what humans supposedly capable, I still unable to surpass the brute force that those 3 beings have. In order to win, we will need the help of all the main cast, but then I faced with the reality that those people might not even be born.

However, thankfully that worry didn't happen.

"There, high! High! Issei!"

"Fufu, Issei sure look happy."

At a small park in the city of Kuoh.

Currently, I hide behind a tree watching a family of three made off a young father, mother, and a newborn baby. That's right without a single doubt the family I currently observe is the Hyodo family that makes an appearance at the LN.

It seems that despite all the thing I did, Issei managed to born safely under his original family, and not just him so far on the list that I manage to confirm Kuroka the Nekoshou already born and currently live at the eastern youkai faction territory. Akeno, Rias, Sona, Sairaorg are also confirmed to be born safely each under their own parent. The Shido family also just welcomed a new family member named Irina.

Frankly, I had prepared that all of them might not even be born, but if it's like this all the cast most likely will be born safely. But I wonder how? I mean, is there some sort of forces that interfering with fate itself ensuring their birth?

Well, that doesn't matter.

In the case they will not be born I had made bunch of preparations and made some allies which I could trust with my knowledge, even so, I still not sure we could win against the invaders. Thus, the confirmation of all the main cast birth is a good thing, hopefully, they grew up properly like the original story and able to help us against the invader.

Though I worry it will go that well.

As I previously said there is some part of the background history that is different from the original story. In that case, it could be assumed that their growth might be affected, if so what choice do I have now… Hmm….

Hn? That was?

Across the street, I noticed a young man walked down the street together with one of the men I have watched over this past 1 year the father of Shido Irina, Touji Shido.

No, who is that young man?

…Hmm… I swear I had seen him somewhere?

Oh, well the fact I can't remember mean he is no one important.

Now, let's see hmm… maybe I should just buy some house here so I can watch it over Issei.

I still need to confirm the birth of several other kids, but frankly, I don't even know where they are so I will leave that for later. Maybe I could even offer my help to train Issei early if I can gain trust from his parents, but well this is going to be much later.