Yagi (八木, Yagi) is Kazuo Matsuda's butler and personal hitman. He has served the Matsuda family for many decades.
Yagi is described as a tall and slim figure donning a black suit. He is around sixty with white hair and glasses. Despite his age, he has remained quite fit, capable of jumping down from the third floor of a building and landing unharmed.
Yagi is a refined and polite individual, always acting properly as a servant. He is proud of his service to the Matsuda family and is extremely loyal, following every order with precision. Any action he takes of his own volition are to benefit the house he serves, as he deeply cares for both the members and status of the Matsuda family. Although he takes care to not let his personal feelings interfere with his work, he does make attempts at reconciling Enokida with his father whenever he is presented with the opportunity, either by making suggestions to Enokida directly or by inviting him to events where his father will be present. According to Enokida, he also has a habit of giving money to Enokida's friends, likely an attempt for them to stay friends with him despite his personality and upbringing.
Eight years prior to the start of the story, Kazuo Matsuda had ordered Yagi to dispose of his son. Yagi brought Enokida to a warehouse and pointed a gun at him, telling him it was just the master's orders. He intentionally missed and let Enokida go, however, providing him with a plane ticket to Fukuoka and promising him that there were people he knew there who could help him. Unknown to Enokida at the time, however, Yagi was following Kazuo's orders in letting Enokida go alive.
After a virus infecting Kazuo's computer threatens his career and reputation, Yagi requests that Kazuo allow him to handle the matter, and he heads to Fukuoka in search of Enokida. He consults several private detective agencies, eventually visiting the Banba Detective Office for assistance in finding the hacker. During his visit, Enokida arrives at the office to meet Xianming Lin. Shocked to find his family butler there, he quickly tries to escape, but Yagi cuts him off by jumping from the third floor window and restraining him. With the assurance that he is visiting Enokida the hacker and not the former member of the Matsuda family, he convinces Enokida to take the job.
The following day, Yagi visits the historic city of Daizaifu. That night, he receives a call from Enokida, asking him to save Kazuki Saitoh, on whose head Enokida has placed and retracted a large bounty. Yagi arrives just in time to save Saitoh from a thug in a Nakasu alley, thanking him for his friendship with Enokida and gifting him with a large sum of money and a Daizaifu souvenir.
With the case concerning the hacker macro-hard closed, Enokida sees Yagi off at Hakata Station. After going through the data macro-hard had collected on Kazuo, Enokida notes that something in Kazuo's past does not make sense, and Yagi gladly reveals that the plane ticket to Fukuoka eight years prior had been bought by Kazuo himself. Several days later, he returns to the Matsuda household and resumes work as usual. On Kazuo's birthday, he is present when Kazuo's computer is hacked with a message wishing him a happy birthday. Yagi laughs as he is told to dispose of the brand-new computer.