Either Bleach crossover or AU where Issei had the holy grail instead of the boosted gear.
Bleach ideas would probably mix themselves in.
It started off young. Issei could see people that others could not.
The first person was a little girl. Her name was Kagura. Hi parents thought nothing of it when their five year old child started talking out to nothing. He was five after all, what five year old didn't have imaginary friends?
No, Issei's parents got worried when he didn't stop.
Issei kept seeing all these wonderful people that others could not see. Most of them became his friends, and he was happy with that! Ms. Calamir always seemed to have a sheet of cookies for the boy, and he loved playing with Freddie and Kylie.
His parents didn't believe him though. Whenever the young boy told his mother and father about his friends, they would always shake their heads and tell Issei none of his friends were real, and that he was eight now. That he needed to grow up.
Issei could not understand. He could see and touch these people, but no one else could. It was weird. There were even times when one of his friends would accidentally break something and he would get blamed for it!
Issei decided. If no one else believed him, then they didn't know what they were missing out on. He stopped talking to his parents about the invisible people when he was about nine.
That was seven years ago.
So, yeah. This idea actually stemmed off from me binge watching the first arc and a half of Bleach. So the idea is Issei with the Holy Grail instead of the Boosted Gear. Some Bleach mixed in, maybe.