Bonds
"Sealing Technique: Crouched Tiger Bullet." The words felt hollow in his mouth as they rang through the silent hall. It won't have worked. That was something he knew before be opened his eyes.
"It's never going to work, Lord Danzo." The assistant told Danzo. "You killed all of Sai's emotions. He will never be able to work the Jutsu."
"So he's worthless then?"
"I wouldn't say that Lord Danzo. He has still attained mastery of a rare style of fighting."
"Fine. Keep him in Root."
It would be years before Sai tried to use the Jutsu again. Almost a different era, a time of war where comrades were made and lost in the blink of an eye.
But the next time those words left his lips they were echoed by the roar of the tiger.
Because the stronger the emotions felt by the user, the stronger the Tiger was.
And the bonds he had made had given him emotions stronger than any he felt before.