Eagle-Masked Anbu Member

He is an Anbu from Konohagakure.

Background
In the anime he, Kakashi Hatake and his partner came to the aid of a Konoha lookout that was getting attacked by a group disguised as Iwagakure shinobi, with the intention to create a rift in the peace treaty between villages. Soon after they arrived, the group retreated leading Kakashi to follow them and mercilessly kill all five of the shinobi before their group could collect intel from them. He later reported back to the Fourth Hokage about the matter and how Kakashi was copping following Rin's death.

During the Nine-Tails' Attack on Konoha, he and a group of Anbu assisted the Third Hokage with dealing with the Nine-Tails after its sudden appearance in the village. When his team launched an attack, they were all easily repelled and defeated by the Nine-Tails.

He and his partner accompanied Hiruzen in infiltrating a secret bunker with the intention to find out who was behind the kidnapping of Anbu and jōnin in the village. After they disarmed the traps, they confronted the person behind the acts and discovered it was Orochimaru. Having been caught, the Sannin attacked the Anbu and Hiruzen and escaped.

Years later, he and his partner stood guard during a meeting between Konoha representatives and the Fire Daimyō. When someone interrupted the meeting, he and his partner immediately restrained the intruder.

Appearance
He has tall spiky brown hair and wears the standard attire of a Konohagakure Anbu member with a sword strapped to his back.

Abilities
He is presumably a powerful shinobi, being an Anbu. In the anime, he was shown to be capable of phasing though solid materials. He also carries a sword hinting he's skilled in kenjutsu.

Chūnin Exams
During the Chūnin Exams, he and his partner went to retrieve Anko Mitarashi in the Forest of Death. After seeing that her cursed seal had resurfaced and hearing that she had encountered Orochimaru, he told her that martial law was to be implemented whenever he was spotted and as such the exams should not continue any further. After carrying her back to the tower in the middle of the forest, their attentions are drawn to a video by Iwashi. They then discuss the progress of the Sand Siblings to the tower, who are unscathed.

In the anime, after Kabuto Yakushi killed the Anbu stationed at Konoha Hospital which were gaurding Sasuke Uchiha, he, his partner, and Yūgao Uzuki randevu'd at the hospital and discussed the matter with the Third Hokage and Kakashi. On the day of the finals for the Chūnin Exams, he was part of an Anbu team that was patroling the border of the village and thought he noticed something but continued his patrol.

Twelve Guardian Shinobi Arc
He was seen alongside his partner watching over Danzo Shimura under Tsunade's orders.

Three-Tails' Appearance
In the anime, he and his partner were part of a Anbu team that was tasked to watch over the Three-Tails. After arriving and relieving Kakashi's group of watch, the group stationed themselves at the lake to write reports to send back to Konoha. While on watch, he and his partner were attacked by Deidara and quickly defeated.

Itachi Pursuit Mission
In the anime while patrolling Konoha's graveyard with his partner, Kabuto managed to sneak past the two and get the DNA of three shinobi from their graves, without anyone becoming aware.

Pain's Assault
In the anime, during Pain's attack on Konoha, he, Yūgao Uzuki and two unnamed Anbu protected Tsunade as she was using Katsuyu to tend to the villagers' injuries. When Pain approached the group, he and the Anbu concentrated chakra to their feet to combat their opponent's ability. When Pain unleashed a large-scale attack, he was caught in the impact but survived and remained by Tsunade's side. After freeing himself and Tsunade from under the rubble, he followed her as she confronted the Six Paths of Pain. After Naruto returned to the village, he asked Gamakichi to carry Tsunade to safety, to which the Anbu followed them and stayed by her side for the remainder of the attack.

Trivia

 * Though he and his partner were believed to be killed in episode 112, they later appear during Pain's Assault.