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 wore a cat-like Anbu mask with red markings on it. He also wore a black hooded cape.
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.
- "Baku" is presumed to be his codename.