Sight
The Sharingan. The all-seeing power possessed solely by those of the Uchiha bloodline. Now that Sasuke was under their sway, Kabuto supposed he should feel some form of relief. After all, Orochimaru was so close to obtaining all that he desired. But there was something about the boy that made him uneasy.
Watching silently from his hiding spot, he observed Sasuke as he trained. Splinters of wood flying in all directions. Trees felled by powerful kicks. Shurikens hitting targets dead center. Branches severed and crashing to the earth below. He would keep this up for hours. Until Orochimaru decided to humour him with a real opponent.
A kunai whizzed past Kabuto's head, the blade missing his ear by a mere fraction.
"So, Orochimaru sent you to spy on me, did he?"
Kabuto emerged from the shadows. Eyes hidden behind the glare of his glasses.
"No, actually. He didn't."
Sasuke cast him a skeptical look. The two faced one another for a brief moment. A small snake slithered past Sasuke's feet. The boy swiftly drove a dagger deep into its body, pinning it to the ground.
"Tell Orochimaru that if he wishes to speak to me, he should come himself. Instead of sending his usual vermin to do the job for him."
He began to storm off before turning back to add:
"And don't think I trust you let alone like you. Feel free to add that to your precious research, Kabuto."
The medic walked over to where the snake lay. Yanking the blade from the unfortunate creature as it writhed in its final death throes. Bitterly, he began digging a shallow grave, using the weapon Sasuke had left behind.
For all that the Sharingan could see, the boy was still clouded in rage. And Orochimaru was too blind with his goals to spot the danger.