What's the secret behind Kakashi's mask in Naruto?

Kakashi Hatake is one of those characters in Naruto that one cannot hate.

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Behind Kakashi's mask

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Kakashi's Full Face Finally Revealed For All the World to See - Interest - Anime News Network

What's the secret behind Kakashi's mask in Naruto?

Naruto: 35 Interesting Details About Kakashi's Body

Naruto Shippuden: Season 17 Kakashi: Shadow of the ANBU Black Ops – A Mask That Hides the Heart - Watch on Crunchyroll

368 episodes later, Kakashi's face revealed in 'Naruto Shippuden

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What is behind Kakashi Sensei's mask? - Quora

What is behind Kakashi Sensei's mask? - Quora