Baku

Baku (バク) was a member of the ANBU, working directly under the supervision of the Second Hokage. He was one of the ANBU ordered to watch the Uchiha clan to keep them from rebelling again. Baku is likely not his real name, but a codename.

Peeking from behind his mask, his eyes were said to be able to perceive everything.

(Source: Narutopedia)