Sumire is a half oni, the offspring of a human mother and oni father.
When Sumire was born, people called her a cursed child and drove both mother and daughter out of the village. Hauling her tired body, the mother went seeking for her daughter's father. However, nothing was there where the oni village once was. A sorcerer than approached the weary mother. Pleading that at least Sumire should have a chance at a good life, the sorcerer coaxed the mother into handing over Sumire. But contrary to what the sorcerer promised Sumire's mother, the real reason why the sorcerer sought Sumire was to use her as a core for a spell. To the sorcerer, the powerful oni blood flowing through Summer's body was the best ingredient. The sorcerer sealed Sumire inside a wooden box and conducted the ritual. However, the power the child beheld was far stronger than expected and it began to overflow out of control. The sorcerer's curse was let loose and it spread all over, taking the life of every living being, including the sorcerer. The area became a land of the dead with only lost souls left. Only Sumire, with the life force of the oni, continued to breathe shallow breaths inside the box. It was Akayuki and Aoyuki who saved Sumire from the sealed box, which not even the lost souls could approach. After purifying the land of evil souls, the two arrived at the core of the spell and found Sumire inside the box. Discovering the child, Akayuki and Aoyuki were dumbfounded at first, but recalling their own past, they decide to take her in as their pupil.
Sumire was raised by Akayuki and Aoyuki since then. The two cared for the child as if it were a competition and this extended to their teaching. Under their attention, Sumire learned various things from martial arts to mystical arts, basic common sense, and the kind of attitude a warrior should have. On the surface, their relationship was that of teacher and pupil. But as time passed, the meaning of their relationship became much more for all three of them.
Nevertheless, on the day of Sumire's last graduation test, this relationship gets ruined. Sumire, who had come a long way, battled the Ancient Demon as a graduation test. At first, it seemed she would prevail. However, at the crucial moment, Sumire let her guard down at the thought of having to part ways with Akayuki once the test was over. The Ancient Demon seized the chance and gathered its last might to complete its Curse of Petrification. To save Sumire, Akayuki intervened and took the curse instead. Even as she was turning to stone, Akayuki didn't blame Sumire. Sumire, on the other hand, who lost Akayuki because of her own mistake, sunk into a pit of guilt. The same went for Aoyuki, who was absent at the time because he had to control the Evil sword.
Sumire and Aoyuki did everything they could to bring Akayuki back to normal, but no existing method could turn her back. After a long period of trial, the two came to accept that Akayuki was really gone. Aoyuki and Sumire still cherished each other, but they couldn't help but think of Akayuki when they were together. Every time they looked attach other they felt guilt towards one another. In the end, as the two couldn't forgive themselves, they decided to part ways.
Sumire, who was now alone, honed the martial and mystical arts Akayuki and Aoyuki taught her, sought ways to turn back the petrification, and helped people around her along the way. Sumire had no particular intention. All she wanted was to keep her promise with Akayuki and reminisce about the previous times when the three were together. Hundreds of years passed like that. Before she knew it, the people she saved, those who admired her and those who emulated her gathered around her. Sumire accepted them and taught them as pupils. They then passed those lessons on and formed a new circle, calling themselves ninjas. As the circle was formed by followers of Sumire, the faith of the "ninjas" towards her was almost religious.
Having lost Akayuki once in the past, Sumire shows a tendency to overprotect her now that she is awake. This was directly passed on to the ninjas following Sumire. If Akayuki even mentions that she's feeling hungry, Sumire and the ninjas go right into action bringing her all the dishes gathered from all the diners in the area. Nowadays, to avoid all this trouble, Akayuki is going out in secret more often. But no matter where she goes, she gets spotted by the ninjas and eventually Sumire tags along behind her.