He had a secret. Multiple secrets, in fact.
Naruto Uzumaki had always been a smart kid. He knew about the nine-tailed fox, who his parents were, and how to execute the basics of ninjutsu as a kid. Thus, the Sandaime had ANBU train him to become one of their own.
Now, at twelve years of age, Naruto the ANBU was given a mission to find promising students and keep tabs on a certain boy at the academy, but that could be a bit hard, given that he's blind.
I don't own Naruto, nor do I claim I own anything having to do with Naruto (only the plot of this story). Masashi Kishimoto is the sole author of the Naruto series and he owns it. That is final.