During the reign of the Second Hokage, he was one of the ANBU ordered to watch the Uchiha clan to keep them from rebelling again. "Baku" is presumed to be his codename.
Peeking from behind his mask, his eyes were said to be able to perceive everything.
A baku (獏) is also a mythological Japanese creature said to eat dreams and nightmares. As a chimera with an elephant's trunk, a rhinoceros' eyes, an ox's tail, and a tiger's paws, they resembled somewhat the real-life tapir. After the two creatures were connected, the Baku was more and more often depicted as a tapir. Modern images depicting Baku as actual chimera are rare.