Log

is a synthetic human created by Orochimaru and the older clone brother of Mitsuki.

Background
Log was created as a synthetic human by Orochimaru, being cultivated from an embryo and given the name, after and the sixth sign of the Chinese zodiac calendar,. However, he rebelled against Orochimaru, escaped and, now calling himself Log, started down the path of destroying the original embryo from which he originated, deeming Orochimaru's experiments to create life unethical. This background story was soon revealed to be a ruse set up by Log and Orochimaru in order to force the young Mitsuki to make a decision: either join him in destroying Orochimaru and eventually themselves, remain with Orochimaru and follow his orders, or choose his own path in life.

Personality
He seems to be a rather calm individual. He also enjoys smoking. Log seemingly has a soft-spot for his little brother/younger clone, smiling lightly from Orochimaru's concern for the boy. Likewise, he was shown willing to aid his younger brother, even at his own expense.

Appearance
Log is a tall individual with dull-blue, shoulder-length hair, and golden eyes. Overall, he bears an exact resemblance to his clone sibling Mitsuki with the exception of a scar across his right cheek. When first seen, he wore a dark-coloured haori, which concealed his entire appearance. When confronting Orochimaru and Mitsuki, he wore what appeared to be a modernised form of a shinobi battle armour complete with large arm-guards, outer thigh-guards and a hybrid breastplate. He also wore a kabuki mask, which had slits for eye-holes, a protuberant nose and a frowning mouth. He takes to using an ornate snake-themed lighter, and carries a sword strapped vertically to his back. In the anime, Log wears a long hooded dark grey cloak.

Abilities
Log appears to be quite skilled as he was able to temporarily fend off and counter his parent's — who himself is a renowned shinobi — attacks for a short while. He also makes use of a sword in combat, showing considerable speed and likewise skill in kenjutsu against his parent. With his Armour-Eater technique, he is able to manipulate his armour into various shapes to aid him in battle, such as encasing and restraining his opponent.

Mitsuki's Disappearance Arc
In the anime, when Mitsuki apparently betrayed Konohagakure to leave with two attackers, word reached Orochimaru. He then met with Log, discussing the continued development of Mitsuki. They began to wonder what was driving Mitsuki now and if their hopes of him becoming a new light to the world would succeed.

Later, it was revealed that Mitsuki was still loyal to Konoha as he helped in stopping the attack of the Fabrications, which Log found a dull ending. Also, following the passing of Ōnoki, an investigation was done on his work about the Fabrications. However, Log soon reported that all evidence was mysteriously removed, leading him and Orochimaru to wonder who could have helped the Tsuchikage in creating such advanced artificial humans besides Orochimaru himself.

Mitsuki came to Orochimaru and Log to discuss his recent memories about Ōnoki and how they began flashing through his mind as dreams. The Sannin scanned his son's mind to research the memories with Log. There, they learned about how Mitsuki went through great personal discoveries about his being, wondering what choices are really his own and if he is on the right path. While Orochimaru was proud to see how much he was maturing into a true human being, he was concerned how the still-raw child was reacting to such personal loss. Log voiced his disapproval of erasing Mitsuki's memories after how far they had come. Ultimately, they gave Mitsuki a choice to continue enduring these painful memories or have his mind reset to before he ever even knew about Boruto. Mitsuki decided to keep his memories and move ahead.

Kara Actuation Arc
In the anime, after pressured into using his Sage Transformation to save his team-mates, Mitsuki's body began breaking down from over-usage. He was brought to Log and Orochimaru for treatment. Orochimaru was dismayed at the severity of Mitsuki's condition, noting that various organs would have to be replaced from scratch. Log noted that Orochimaru was very concerned for Mitsuki, who simply said that he has invested so much time and resources into creating Mitsuki, not wanting it to be a wasted effort.

Later, as Mitsuki began slowly making a recovery, Log investigated and learned that the Hashirama Cell Kara acquired was actually stolen from one of Orochimaru's old laboratories. When Log asked what to do next, Orochimaru simply said not to bother, as the First Hokage's genes weren't easily harnessed. As Mitsuki's replacement organs were almost viable, Suigetsu returned with news of Mitsuki's team. He revealed they all returned to the Land of Valleys. While the genin and jōnin respectively went for different reasons, Suigetsu suspected that there was a connection between the two. Mitsuki was determined to help his team despite his still unstable state, forcing Log to sedate him.

Ultimately, to help his younger brother return to the battlefield quickly, Log allowed Orochimaru to use his own organs as a substitute for Mitsuki, ultimately swapping positions with Mitsuki as Log was put on life-support to wait for the replacement organs.

Trivia
Log Log Log Лог
 * In a censored version of the anime, Log's cigarettes are edited out. Instead, it only shows him turning on a lighter, implying a like of fire.