Uchiha Obito or Uchiha Itachi?
It seemed like a simple choice, but it wasn't.
However, neither was the best option for Uchiha Rei.
Kotoamatsukami's true nature wasn't simply to confuse someone with Genjutsu. It forcibly implanted a new Thought Seal in the target's mind, making them act according to the caster's wishes without realizing they were being controlled.
In theory, it sounded nearly perfect. In practice, it wasn't very useful.
Historically, the only people successfully controlled by Kotoamatsukami were Mifune and Uchiha Itachi. Even so, its effect on them was...
So-so.
Kotoamatsukami didn't create a new will. It amplified vague thoughts already present in a person's mind.
If it were used on Uzumaki Naruto and ordered him to "destroy Konoha," the result might just be that he changed the village's name.
Those under Kotoamatsukami's control interpreted its "commands" according to their own logic.
That was why the more normal someone was, the more easily they could be influenced by Kotoamatsukami. Clear thinking, sound logic, and straightforward emotions made someone an ideal target.
Uchiha Itachi, however, was anything but normal. He was a notorious Uchiha lunatic.
His will was full of twisted beliefs and deranged logic. He believed his younger brother was his entire world and was willing to sacrifice the whole Uchiha Clan for him. If Kotoamatsukami went against that supreme conviction, his subconscious would twist its effects.
Uchiha Obito wasn't the best choice either. He was still trapped in his own Yellow Springs Space, and Uchiha Rei knew nothing about his current condition. His Genjutsu clone still hadn't sent back any information.
Perhaps the moment he opened Yellow Springs Space, Uchiha Obito would escape through Kamui.
Both men wielded the Mangekyō Sharingan and had resistance to Genjutsu beyond that of ordinary people. The odds of controlling them with Kotoamatsukami