Let's rewind time a bit.
In the deep, silent forest, trees towered exceptionally high, the air crisp and cold. Aozaki Touko left a trail of footprints in the snow, moonlight dappling her body through the gaps in the leaves.
She had walked this path countless times, and often at night.
In the past, because she wanted to balance her Magecraft research with spending time with Aoko, she would sometimes choose to complete her research at night and rush back to the Aozaki Family under the cover of darkness to be with Aoko.
Therefore, she was very familiar with this path.
Soon.
She arrived in front of her grandfather's Magecraft workshop.
This was a cave recessed into the mountain, lit by lamps. One could vaguely see many bookshelves inside, laden with books in various languages.
It was as if a library had been carved out inside a mountain cave.
"...Still as gloomy as ever."
Whispering softly, she stepped into the Magician's workshop, walking to the table where she used to read grimoires. Her grandfather, with perfect understanding, stood there waiting.
Just like when her grandfather was about to teach her in the past.
The Magician's workshop was silent.
"..."
Aozaki Touko did not speak first. Instead, she picked up a grimoire from the table and patiently sat down to read it, just as she used to practice here.
The grimoire in her hand was one she had left behind when she departed.
And this grimoire seemed to have been frozen in time; not a single page had been turned since she left, maintaining the exact state she had left it in.
It was as if a broken timeline had reconnected.
"..."
Her grandfather did not mind Aozaki Touko's rudeness.
His gaze was devoid of any emotion, as if he had transcended human understanding, staring coldly at Aozaki Touko, as if observing a lifeless object, judging something from the purest angle of benefit.
After a long while, her grandfather finally spoke.
"Aozaki Touko, do you wish to inherit the Fifth Magic?"
His words were slightly more serious, tinged with a subtle emotional nuance.
His address also changed from 'Touko' to her full name, 'Aozaki Touko'.
This was a rare display of emotion from him.
"Am I not the heir to the Fifth Magic?"
Aozaki Touko paused slightly, her hand flipping through a book, then casually countered.
Almost everyone in this world, including her parents and Aozaki Aoko, believed that Aozaki Touko was the future Fifth Magician.
Even… when she was young, many Magicians visited her.
All of them came to witness her, the future Fifth Magician.
"If you wish to inherit the Fifth Magic, then the Fifth Magic will certainly be yours. However, before that, you must engage in a struggle with Aoko to contend for the right of inheritance."
Without answering Touko's question, Aozaki Grandfather merely declared.
"Grandfather, is this a sudden whim of yours?"
Closing the book in her hand, Aozaki Touko's expression was calm as she countered.
Regarding the matter of having to fight her sister for the right of inheritance, she showed no anger, merely asking Aozaki Grandfather why he had suddenly changed his mind.
"…I thought you already knew the reason."
Aozaki Grandfather let out a cold laugh, then spoke to himself, "Except for the very beginning, you were progressing according to the steps of becoming a Magician. But other than that, haven't you always been pursuing your own path?"
"Why, when you could inherit the greatest miracle all Magicians strive for, do you insist on following a divergent path, when the correct path is right before your eyes?"
"Or have you already guessed something?"
The cold voice simply narrated, without a trace of questioning emotion mixed in.
In fact, Aozaki Grandfather didn't care that Aozaki Touko deviated from the path of a Magician; he didn't even try to correct Aozaki Touko's thoughts, merely teaching and answering Aozaki Touko's questions as usual.
Then, he watched to see how far Aozaki Touko could go.
And then, he received an unexpected answer.
The young girl, acting according to her own will, achieved an unimaginable success, now standing at the pinnacle of the mysterious world. Her research results over just a dozen years had already surpassed those accumulated by several generations of Magician families.
If he hadn't been a lucky one to enter the Root.
Then, this young girl's achievements would have been enough for him to look up to for a lifetime, unable to even contemplate pursuing, because it had already surpassed the realm achievable through mere effort.
Therefore, he hesitated.
Should he choose Aozaki Aoko, who had a better affinity with magic, or Aozaki Touko, who was a collection of talents?
If he chose Aozaki Aoko, the Fifth Magic could be best utilized. If he chose Aozaki Touko, the Fifth Magic could be best developed.
No matter which person he chose, a good outcome could be achieved.
But, there was always a better one, wasn't there?
Such a tangled thought was born.
A very greedy idea, but for him, a pure Magician.
Wanting the inherited Magic Crest to have a better host.
Isn't that simply a matter of course?
"Indeed, Grandfather never intended to give me the Fifth Magic from the beginning, did he?"
Aozaki Touko turned her head, her gaze turning cold, "But why the sudden change of mind now, and at a time when I'm making progress!"
"Everything in this world is determined by value. The current Touko possesses higher value, so it's only natural to change the decision, isn't it?"
Aozaki Grandfather replied.
"…What a disagreeable answer."
Aozaki Touko sighed, then asked, "Then why do you want Aoko and me to fight to the death? There's no fairness in that at all!"
"Nor can it accurately measure our value."
Even without personally using her own power, she could very easily kill Aoko just by relying on the forces she had gathered.
The simplest method would be to have Kiritsugu Emiya hunt Aoko; just one bullet from a distance could easily take Aoko's life.
There would be absolutely no difficulty.
Unless Aozaki Grandfather intervened to play favorites, or Kujou Alice came to the rescue.
Otherwise, Aoko would have absolutely no chance of winning.
"I just want to see how far you're willing to go for the Fifth Magic."
Aozaki Grandfather said coldly, "If you can erase unnecessary emotions and fully devote yourself to the research of the Fifth Magic, that would naturally be the best thing for me."
Unlike Aoko, who was merely a talent to be utilized, he needed Aozaki Touko's research talent to be focused on the Fifth Magic as much as possible. If she could abandon everything to develop and research the Fifth Magic, it would naturally be an excellent choice for him.
That's right, it was such a simple reason.
Only one heir to the Fifth Magic was needed; even if the other possessed great talent, it wouldn't mean much, and her death would not be worth regretting.
If Aozaki Touko was willing to abandon everything, existing solely for the Fifth Magic.
Then entrusting the Fifth Magic to her would not be out of the question.
"…Killing a sister to prove one's path, huh."
Aozaki Touko chuckled softly, shook her head slightly, then asked, "Since Aoko's priority in inheriting magic is above mine, then why did you still choose to have me learn magic?"
This question was something she wanted to know in her previous life.
Even in this life, she remained puzzled by it.
"I thought you had already figured it out."
Aozaki Grandfather smiled coldly, then began to explain, "Fruits from the same tree are difficult to distinguish before they ripen. All I want is the most perfect fruit."
"So, after discerning your respective qualities, I chose to bring you here for magical training, to allow your talent to flourish. If there were unexpected results, then I would include you in the consideration for the Fifth Magic heir."
"If you merely grew according to the path I envisioned, then you might as well serve as a whetstone for Aoko to become a Magician."
"It's not a bad thing either way, so why wouldn't I choose you to study magic?"
Cold words echoed within the cave.
These were answers from a Magician's perspective, prioritizing the inheritance of the Magic Crest. Thinking with this logic, Aozaki Grandfather's thought process could be explained.