Alex Strife
Ex-SOLDIER
Madara seems to be developing a certain fixation on Onooki's resolve and a certain need to break his will.
Because they knew each other, and because Oonoki's kind of the leader of the Kages now. Not because he's above them, but because he was the one who trusted them the less and he learned to trust them, to work alongside them, and he's giving himself completely to the Shinobi Alliance.
Also, his combat experience means he knows how to fight a bit more effectively than the others. That's why his will, his resolve, is so important.