In that case, why don’t I try getting to the heart of the Kuruokas’ magecraft? The child Tsubaki must be rooted in it.
“What type of magecraft do you study?”
The expression vanished from Kuruoka’s face as he answered.
“You expect me to tell an outsider that?”
It was what any mage would say.
At the Clock Tower, you could tell the direction of a mage’s interests by the school they belonged to, and many announced their work to gain influence. Even so, few would disclose the specifics of their magecraft. That practice was not limited to the world of magecraft; ordinary businesses and researchers did the same thing.
Nevertheless, Sigma deliberately pushed ahead to confirm his guess.
“I want to know so that I can secure Tsubaki’s safety.”
He did not lie.
Sigma’s current objective was to escape from this ward-world, but before being sent there, it had been to accompany Assassin and secure Kuruoka Tsubaki’s safety.
He did not know what kind of abilities that pitch-black Servant possessed, but if it had the power to detect lies or hostility, deceiving Yūkaku could prove to be fatal.
More importantly, his question was also intended to confirm something.
Kuruoka Yūkaku’s eyes glazed over for a moment, then, after a few seconds, he broke into a placid smile.
“I see. If it’s for Tsubaki, then I guess I’ve got no choice.”
With that, Sigma was certain.
As I thought, this world, including the personalities of the people being controlled, exists to protect its Master. That pause must have been the time it took for the Servant controlling Kuruoka Yūkaku’s mind to make a decision and guide him.
And it’s probably not the type to doubt what I say, as long as I don’t lie.

Fate/Strange Fake: Volume 6, Interlude ("A Mercenary is a Free Man I")