Is that the uchiha crest on kawakis back?Yeah his indepth anlaysis is nothing to do with deduction based upon observance. Even Borat concluded that he's not oridinary and said that's obvious yet Kakatsuke went further to confirm and describe how and what Kawaki is on the insides. That doesn't come from deduction or observation rather from Katasuke's glasses or you know 123's least favourite words(bad writing).
Edit:
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Katasuke's clueless during the battle.
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Here's Katasuke after getting close to Kawaki, looking at him and telling us exactly what he is.
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He even knows the boys different to Mitsuki and was initially human.