His given name when born is actually Samuel T. Owen.

As a child, Samuel was seen ruthlessly killing numerous animals in cold blood for entertainment in a barn, much to his father's fear. While his father scolds Samuel and loves him deeply, Samuel himself seems indifferent and incapable of understanding his emotions.

Satō was part of the United States Marine Corps in the past. Nicknamed "Poker-face," he began his 8-week recruit training in San Diego in 1969 by faking his age to join the corps. After the first 2 weeks, he left because of a supposed serious illness. This was, however, a cover, as he was actually recruited to be part of an ultra-elite section of the military known only as the "team" while fighting in Vietnam.

In 1976, after the end of the war, Satō and the "team," as well as a nameless man being questioned by Tosaki in present time, are assigned to a secret recovery mission in Vietnam. With Satō in the lead, the group had managed to perfectly execute the mission in 2 days without firing a single shot, but just as the team was to return to the extraction point, "the devil woke up," as Satō fired a shot, catching the attention of hundreds of enemies, forcing the men into a battle, resulting in one member dead, one heavily injured, and Satō himself without a leg. After his return, he was dishonorably discharged.
After he was discharged he spent most of his time playing arcade games until he was recruited by his uncle and went to Japan. Sometime later his uncle was killed by his rivals. Satō went on to make a lot of enemies in the underworld and was executed however he immediately revived discovering he was an Ajin.
Satou's first revival

Satō being revived for the first time.

Just before he met Kōji Tanaka during one of his jobs he took a hard drive from a researcher of the Ajin control commission and learned what the Ajin control commission really was.

The chapter begins with a flashback showing Sato being abused by his father when he was a child.

After traveling by plane Tosaki meets a former marine in order to learn more about Sato's true nature. Carter tells Tosaki about how he first met Sato in 1969 when training to become a marine. Sato was soon pulled out to join an elite team of soldiers. Carter met Sato again in 1976 in a mission to save a POW in Vietnam. They rescued him but during the mission Sato purposely fired a bullet to summon countless of enemies. This resulted him in losing his right leg and was dishonourably discharged.

Meanwhile Tanaka kills their 5th target thanks to information given to them by a police officer.

Tosaki meets with the rest of the group and confesses that the minister caved but they were still going to fight Sato regardless.

Tanaka returns to Sato to inform him that they killed the 5th target and asks who they should target next. Sato confesses to Sato that he is bored of the list and that Tanaka should take care of the rest without him. Tanaka meets up with Okuyama, who gives him the blue prints to the forge security building.

While Kei is being driven Kei tells Nakano that people don't change. The story shifts back to the the flashback of Sato's childhood where he was hit by his father. Sato's father tells Sato that life is precious and that he loves him after he caught Sato killing animals