Ushi

was a member of the ANBU, working directly under the supervision of the Second Hokage.

Background
He was one of the ANBU ordered to watch the Uchiha clan to keep them from rebelling again and he did so vigilantly. Ushi is likely not his real name, but a codename.

Abilities
As an ANBU, he was one of Konhagakure's strongest shinobi. The fact that he was tasked with the very important and difficult mission to observe the Uchiha clan furthermore implies that he was very powerful.

Trivia

 * "Ushi" means "cow".