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.

Appearance
Ushi wore a black Anbu cloak and a white Anbu mask over his face with a big triangular purple mark on the mask's forehead area.

Abilities
As an Anbu, he was one of Konohagakure'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". It is likely not his real name, but a codename.