Disclaimer: I don't own Highschool Dxd except for the original characters that have made. This story takes place during the second season of the anime/ volume 3 of the light novel series.
Prologue
(...A young girl stares out the window as she reminisces her childhood.)
"Sei-chan! Sei-chan! Promise me we won't leave each other's side no matter what?"
"Mhm! I promise Mari-tan! We will always be best friends." A boy with brown hair makes his vows to the young girl."
As they both smile and chuckle, the boy offers his pinky to her.
Out of curiosity she questions him on what he's insinuating.
"Here, if we're going to make a promise then we need to do a pinky swear!"
"Ahhh! That's amazing! Okay let's pinky swear each other then."
Their pinky intertwines with each other and they both lock it as a symbol of their never ending friendship.
...
Mariko Higurashi, currently in the second year of high school has moved out of Kyoto to return to her hometown Kuoh.
Her foster mother has decided for them to come back because of two reasons; Mariko and Kokoro have been homesick for the past two years of them residing Kyoto, thus as a business woman, her company is in need of her help again once more.
It's been awhile since Mariko has made contact with Issei. It's not like she's forgotten about him. She has been busy recently with finishing school and packing.
Issei, on the other hand, he has been going through a lot ever since he became a devil. His job as a Gremory's servant, he doesn't have any time to make contact with Mariko that much. However, on rare occasions, he does keep in touch with her every now and then if he has the time to.
He's not aware of his best friend's return because she intends to keep this as a surprise.
That's not the only surprise for him….
There will be also unpredictable events that both of them are destined to happen.
What will it be?