Ever since he was a teenager, Kotaro trained with Issen the Master in the Japanese countryside to become a professional assassin. However, when he moved to Tokyo and joined a Tojo Clan family as a hitman, he found that he couldn't actually stomach killing a person and thus failed at every job he undertook for the family. Eventually, his family's patriarch grew tired of him and had him expelled.
Desperate for work and unprepared for civilian life after training in combat for so long, he joined the Reapers, who would pay him to take out yakuza. However, Kotaro still couldn't bring himself to kill anyone, so he instead spent his time shaking down low-rank yakuza on the streets by using his training under master Issen as a threat; something that proved remarkably effective and allowed him to make money without killing anyone.
After he joins forces with Ibuki and the Honest Living Association, Kiryu is sent to find Kotaro and convince him to leave the Reapers. Kiryu tracks Kotaro down to West Park, where he finds him mugging a petty crook. After Kiryu interrupts his shakedown, Kotaro tries to intimidate him into paying him, but Kiryu is unimpressed by Kotaro's boasting.
When he finds that he can't actually shake Kiryu down, Kotaro flees, prompting a chase through Kamurocho. Eventually, Kiryu stops Kotaro and, in desperation, the Reapers' member attacks his pursuer with a stun gun. Kiryu defeats Kotaro and convinces him to join the HLA and find work as a civilian.
As a reward for recruiting him for the HLA, Kiryu is paid 70,000 yen.
In the substory "Kotaro Gets a Job", Kiryu runs into Kotaro once more right after he gets fired from "Kamuro Ramen"; a restaurant next to Bantam, for getting into an argument with a customer. Despite being upset, Kotaro brushes off his firing and heads back to the HLA headquarters to visit Ibuki and start looking for another job. As Kiryu learns after having a chat with Ibuki, this is not the first time that Kotaro has gotten fired for arguing with a customer.
Some time later, Kotaro has found another job working as a waiter at Kanrai. Kiryu runs into him again, this time mid-argument with dissatisfied customers outside the restaurant. The confrontation escalates into a fight and Kiryu intervenes, helping Kotaro fight off the disgruntled patrons. Unfortunately for Kotaro, the restaurant manager saw everything and fires him for fighting with clients. After Kotaro relays the details of what led to the fight, Kiryu teaches Kotaro how to be more polite to customers who are acting rudely so that they'll stop or leave without escalating to an altercation.
More time passes and Kiryu finds Kotaro lying injured on the street outside Don Quijote, surrounded by the angry customers from Kanrai. Kotaro had gotten a new job apprenticing to be a sushi cook at Sushi Gin when the same customers came into the restaurant. The angry clients attacked him again because of their previous fight, but this time Kotaro did not fight back, maintaining a polite customer service attitude. The thugs attack Kiryu to take revenge for his role in the previous fight, but Kiryu easily dispatches them with Kotaro's help, who now does fight back at Kiryu's insistence. Immediately afterwards, Kotaro's boss arrives and, despite what Kotaro initially believes, he does not fire him. The boss saw how Kotaro tried to be polite even when he was attacked and how he fought when given no other choice and is impressed with him. Excited, Kotaro thanks Kiryu for his help and goes back to work,