Zō

He is a captain in Konohagakure's ANBU force.

Invasion of Konoha Arc
The ANBU captain and seven other ANBU members were stationed in the Chūnin Exam arena during the final rounds of the Chūnin Exams. When Orochimaru launched the invasion of Konoha, the ANBU captain and his team rushed to help the Third Hokage. However, before they could reach him, the Sound Four put up a barrier that kept them from helping. He and his men were forced to watch the battle between the Third Hokage and Orochimaru. In the end, the Third Hokage died due to using the Dead Demon Consuming Seal and Orochimaru stabbing him with the Kusanagi Sword: Long Sword of the Heavens. When Orochimaru retreated, the ANBU captain and his men tried to prevent them from escaping, but Kidōmaru prevented them from following with his webs. At the Third Hokage's funeral, the Captain cursed his inability to help the Hokage, saying "In the end, all we could do was watch".

Trap Master Arc
In the anime, a similar-looking ANBU captain and his team interrogated Naruto about Gennō by using the Time Reversal Technique.

Three-Tails Arrival arc
In the anime, a similar-looking ANBU captain was badly wounded by Kabuto on a mission trying to find the location of one of Orochimaru's hideouts. He sent a messenger bird back to Konoha with information about his mission before destroying his body.

Invasion of Pain Arc
Another identically dressed ANBU captain stayed by Tsunade's side during the beginning of the arc, and later accompanied Ino when she went to aid her father and Ibiki Morino. This ANBU captain defeated one of Pain's summons using Lightning Release: Four Pillar Bind with the aid of two other unnamed ANBU.

Trivia

 * This ANBU captain has been given the name Otomatsu by fans and the closed captioning. He has also been called Raidō, possibly because he was confused with Raidō Namiashi.
 * The other ANBU were once seen wearing black cloaks, while their leader wore a white cloak giving a distinction between their leader and them.
 * There have been several appearances of an unnamed ANBU captain from Konohagakure, wearing white robes and a cat-like mask. It is unknown if any of these appearance are the same person. In the anime, there are at least two of them, as the second to appear died shortly afterwards.