The events of that day are burned into the memories of the young 'girls' with an intense impression.
At the time, they were three years old. The twin sisters—the younger one branded with the label of 'not awakening a decent technique' because she had become aware of it, and the older one who hadn't awakened to cursed energy at all—were being dragged to the cursed spirit room by their 'father' as usual, intending to discipline them out of anger.
"Hello there. What are you doing, Ogi-san?"
The voice of a boy carrying a unique rhythm. Perhaps having just gone through puberty, his voice beginning to show the depth peculiar to men, the boy frowned without trying to hide it at the 'father' he happened to pass in the hallway.
"...Tsk, Naoya."
"Are you listening? I asked what you're doing. Answer me."
"...I am simply educating my failure of a daughter."
"Is that right? That's strange, considering where you're heading. Isn't the cursed spirit room up ahead?"
"It has nothing to do with you!"
The father's obvious displeasure. The symbol of their fear. However, on the face of the boy confronting him, there was absolutely no fear; rather, what surfaced was 'anger' and 'disgust'.
Immediately after he let a glimpse of 'killing intent' ooze out, it was their father, 'Ogi Zen'in', who moved first, tossing his daughter aside.
Although he didn't draw his sword, the fist imbued with cursed energy was unmistakably laced with lethality. Even though training that bordered on killing each other was an everyday occurrence in the Zen'in household, the blow contained a level of power that was arguably excessive. It flew towards Naoya's face without missing its mark—but it did not hit.
The moment Naoya caught that fist with his right hand, a strange rigidity occurred. Ogi's body turned into something like a 'film' in an instant, halting in defiance of acceleration like a 'still image'.
Naoya, who slammed a 'spinning back kick' into Ogi in a counter-like motion and kicked him out into the courtyard, spilled words along with a sneering smile.
"Are you an idiot? You're gutter trash who can't even beat Papa, so why did you think you could beat me?"
"You...!"
"Anyway, knowing my 'Binding', you try to discipline your daughter in the hallway? You're so stupid I'm worried about your level. Wanna go to a brain hospital right now?"
Even while provoking him, the exchange of blows was unreasonably one-sided. Every time Ogi tried to punch Naoya and had his blow caught by Naoya's right hand, he turned into 'film' and froze, while Naoya's attacks unilaterally beat down the frozen and defenseless Ogi.
In addition, every time Ogi took an attack from Naoya, he vomited blood as if he had been 'punched multiple times'. Clearly, victory was in Naoya's favor.
"Pathetic. Are you really a Grade 1 with that? There's a limit to being lackluster."
"...Gh!"
Naoya provoked, provoked, and provoked him relentlessly, and Ogi went mad with anger at the humiliation. Amidst this, Ogi finally drew his sword, and crimson flames swirled around him.
"Technique Release, 'Blazing Courage'!"
"You snap just because I said you're lackluster? Even kids play with fire, you know?"
Using Naoya's provocation as a trigger, the blade of roaring flames was swung. However, before that blade could reach him, a 'clump of air' launched by Naoya's fist, which had already swung through, smashed directly into Ogi accompanied by the sound barrier. It knocked down everything, flames and all, and embedded Ogi's body deep into the ground.
"Be grateful. I decided not to kill you. Leave these kids and go to sleep already."
Naoya told this to Ogi, who was covered in wounds and no longer had the strength to speak. Snorting as if amazed, he walked over to the 'girls' who were collapsing in the hallway, still dumbfounded.
"Maki-chan and Mai-chan, right? Come here. I'll talk to my old man and handle it."
What appeared on his face as he said this was an expression that seemed to say 'this is truly a pain in the ass', which didn't match his own words at all.
Conversely, to 'them', that looked far more sincere than the lot who feigned strange kindness.
Thus, on that day, it was declared by the single word of the Zen'in clan head, 'Naobito Zen'in', that 'Maki Zen'in' and 'Mai Zen'in' would enter bridal training as fiancées of 'Naoya Zen'in'. 'Ogi Zen'in' accepted the 'betrothal money' paid from Naoya's personal funds, and in exchange, was forced to enter a binding that prevented him from bringing any harm to the twins.
Behind the scenes of such events, there was Naoya Zen'in, who was grumbling endlessly over the phone to 'Toji Fushiguro' with a truly fed-up face: "Don't screw with me, I'm not a lolicon, you shitty old man! What's so happy about marrying cousins? I want a wife who's got curves. Like Special Grade Tsukumo. I want to cry. What's sadder than taking four-year-olds as wives? And two of them at that." He was being brushed off curtly with a "Shut up, lolicon," but that is a story for another time.