Mukai Kohinata

was a jōnin and Anbu of Konohagakure.

Background
Mukai's family split off from the Hyūga clan several generations before his birth. Many assumed this meant his bloodline was too diluted to possess the Byakugan, but Mukai did in fact have a Byakugan in his left eye; he preferred to keep this a secret from anyone he wasn't close with. Once he became a Konoha-nin, Mukai led an accomplished career, ultimately joining the Anbu at the same time that he was promoted to jōnin. Although his membership in the Anbu had to remain a secret from most, Mukai's performance was nevertheless stellar enough to earn the trust of the Third Hokage. Mukai eventually married and had two children. But when one of his children, a son, became sick, Mukai struggled to afford his treatment. In order to make the needed money, he began spying on Konoha for Kirigakure.

Mukai was part of a team assigned to provide security to the Land of Fire's daimyō during his annual visit to Konoha: Konoha's Team 2 publicly served as the daimyō's escorts, while Mukai and his Anbu followed in the shadows to monitor for threats. When the daimyō's entourage was confronted by the masked man, Mukai and most of the other Anbu fell prey to the man's genjutsu. Only Kakashi Hatake resisted the genjutsu, but it took him some time to free Mukai and the others. They went to the daimyō's defense as soon as they were able, but the masked man had already been driven off by Itachi Uchiha.

In addition to his Anbu duties, Mukai also went on regular missions. For one such mission, a few years after the incident with the masked man, Mukai's team was to infiltrate Sunagakure and monitor a meeting between the Fourth Kazekage and a representative from Kirigakure, whose villages were secretly developing an alliance. Such an alliance would break the delicate peace that had been formed after the conclusion of the Third Shinobi World War and inevitably initiate another war. Mukai's team was to gather information so that Konoha could take the necessary steps to prevent this from happening. Although they went unnoticed during the reconnaissance itself, one of Mukai's teammates set off a booby trap as they were leaving, alerting Suna's ninja to their presence.

Mukai and his team rushed from the village trying to cross the border back into the Land of Fire before the Suna-nin could catch up to them. As they traveled, however, Mukai became convinced that the Suna-nin would catch them anyway, so opted to stop and wait for them; Mukai drank some sake and smoked a cigarette to pass the time. The Suna-nin soon reached them and surrounded them. Itachi Uchiha, temporarily added to Mukai's team after one of the usual members was mysteriously injured, assumed he and the rest of the team would be joining Mukai in fighting the Suna-nin. The other members, knowing Mukai's thinking, instead pulled Itachi away from the fight so that Mukai could fight them alone. Mukai activated his Byakugan and proceeded to engage the more than twenty Suna-nin in close combat, defeating them all in quick succession. With their pursuers dealt with, Mukai and his team resumed their course back to Konoha. (In the end, because Suna and Kiri's planned alliance had been discovered by Konoha before it could be formalized, the two villages were forced to abandon it.)

On Mukai's next day off, he created a shadow clone to take his place around Konoha, notably spending some time with his sick son. The real Mukai, meanwhile, met with his Kirigakure contacts. Itachi and Shisui Uchiha, assigned by Danzō Shimura to eliminate Mukai for betraying Konoha, covertly followed Mukai to the meeting, defeated all the Kiri-nin he was meeting, and then confronted Mukai. They encouraged him to surrender, but Mukai declined, knowing he'd betrayed too many of the village's secrets to receive any mercy from Konoha's government. He released his shadow clone back in Konoha in order to regain its chakra, mused he would need to come up with an explanation to tell his son, activated his Byakugan, and then attacked Itachi and Shisui.

Despite Itachi's attempt to keep his distance, Mukai quickly got close to him and performed a disemboweling maneuver, but the Itachi he attacked turned out to only be a crow clone. Shisui then attacked him with a Great Fireball, but Mukai dispersed the flames with his Gentle Fist. He began to spar with Shisui, but soon noticed that Shisui seemed to be guiding him towards something. Guessing that Itachi had concealed himself somewhere in order to catch him in a genjutsu, Mukai discretely located Itachi's hiding spot so that he could avoid it, then attacked when the opportunity presented itself. Shisui attempted to cut Mukai off, but this was only a shadow clone, the real Mukai having snuck up on Itachi from behind. Mukai began striking Itachi's tenketsu, but was interrupted by an attack from Shisui, directed at the Byakugan's blindspot.

Mukai grabbed Shisui by the throat and started to strangle him. Though Itachi was having difficulty moving because of Mukai's previous attack, he nevertheless tried to intervene, approaching Mukai with his Sharingan active. Muaki warned him that the Sharingan's genjutsu wouldn't work on him, prompting Shisui to wonder if the same was true of the Mangekyō Sharingan. During the brief instant that Mukai made eye contact with Shisui's Mangekyō, he was captured by its genjutsu. He released Shisui and in the same moment cut open his own stomach, the result of a jutsu meant to prevent him from being interrogated if and when his spying activities were ever discovered. Mukai inquired what the Mangekyō Sharingan was, being unfamiliar with it, but received no reply.

With his wound too severe for him to be saved, Mukai confessed to his actions, taking full responsibility so that his wife and children wouldn't be punished for his actions. He attempted to get a final cigarette for himself, but his motor skills had already deteriorated too much. Itachi placed a cigarette in his mouth for him, which Shisui lit. After savoring the cigarette for a moment, Mukai told them he'd be waiting for them on the other side before finally dying.

Personality
Mukai was a smoker as well as an alcoholic and would indulge in these vices whenever he had the opportunity. Despite this, Mukai was a capable ninja and confident in his abilities, even willing to compromise the mission in order to display his abilities for the famed Itachi Uchiha. In battle, Mukai showed that he could be merciless, attacking the leader with a full Eight Trigrams technique, despite the fact that a single blow would have sufficed, not wanting to let the leader of the enemy team escape unscathed.

Abilities
Mukai was an extremely powerful and experienced shinobi, credited by Danzō Shimura as being similarly talented as Itachi Uchiha, though not with the same innate skills as Itachi. He was apparently in the habit of not only facing superior numbers of enemies by himself, but moreover of prevailing against them unharmed, as his teammates knew to leave him to fight by himself without being told. He was confident that he would defeat Itachi and Shisui Uchiha, despite recognizing them both as prodigies in the Uchiha clan, and in fact only lost to them because he did not know about the Mangekyō Sharingan.

Byakugan
Being distantly related to the Hyūga clan, Mukai was able to activate the Byakugan in his left eye, a genetic throwback. His eye only resembles a Byakugan when it's active, and otherwise it appears like any normal eye. The Byakugan allows Mukai to see chakra, helping him to precisely attack his opponents' tenketsu as well as locate enemies that are trying to hide from him. He is well aware of the Byakugan's blindspot, and as such has trained to compensate for it, specifically by conditioning himself to immediately react to any attacks directed at his blindspot in order to minimize damage and begin a counterattack.

Taijutsu
Mukai's Byakugan gives him a huge advantage in combat, allowing him to see everything in his surroundings and react with maximum efficiency: when a volley of kunai is thrown at him, he only bothers to deflect the kunai that would have hit him. Because the Byakugan can see the chakra pathway system, he is similarly able to focus his attacks at where opponents are most vulnerable, typically defeating them with only a single strike; this is known as the Gentle Fist. Although the Gentle Fist is most commonly employed against opponents, Mukai can use its same principles on opponents' jutsu, carefully cutting a Great Fireball in two so that it passes by him harmlessly. Though he's not a member of the Hyūga clan, he does at least know how to perform some of their lesser Eight Trigrams skills.

Even among those who don't know about his Byakugan, Mukai is feared for his taijutsu prowess, as he's both incredibly fast and incredibly strong: he could quickly catch a retreating target, plunge his bare hands into their stomach, and pull out their intestines in order to disembowel him. Because his reliance on close-combat makes him naturally vulnerable to long-range attacks, Mukai is careful not to let his opponents get too far from him so that they won't gain an advantage over him.

Trivia

 * literally means "small sunny place" and "mukai", when written as "向い", means "facing".
 * Reversing the kanji of, one gets 向日, which can be read as "mukai".