Historical ninjas, as the majority of people imagine them, are purely fictional. That kind of ninja has never existed, the way Hollywood and video games portray them.
But Kakashi, Itachi, and Danzo best represent that ideal.
For the most part... Ninja's weren't godly assassins. They were more closely associated as spies or informants. Often wearing civilian farming clothes than actual Black Ninja clothes.
They'd be more like the people who'd wander into a village, and during the night open the gates from the inside. Or perhaps start a fire to distract an enemy so that an allied force could assault the enemy team.
Kuro Zetsu
1.
Stealth. He's the only character who consistently uses stealth. His introduction is spying. His major scenes up until now were spying. Even after being "defeated" by Chojiro, he was secretly hiding under the current battlefield.
2.
Power. He's not some god who whips out giga-scale destructo disks/giant chakra entity armors/partical disassembling beams. He just has himself really. He has Mokuton, but even that wasn't large scale like Madara or Hashirama (or even Yamato).
3.
Tactics. He actually thinks about what he does. Not as strong as some of the others?! Take a hostage (Obito). Help Madara and Obito and perfectly conceal his ultimate allegiance.
The only things that detract him from, "Being a perfect ninja," is 1: He's not even truly human. He's a manifestation of will. 2: He uses Mokuton and can blend into the earth. That's about it though.
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