Shunsuke Saruwatari (猿渡俊介, Saruwatari Shunsuke) is a top assassin formerly working for Murder, Inc. He relocates to his hometown of Kokura in order to fight tougher opponents and teams up with his old friend Naoya Nitta. After making a name for himself in the Kitakyushu area, he is dubbed the Submarine Ninja for his outfit, weapons of choice, and throwing style. He is a member of the Kokura Fried Wings baseball team (Ogura Fried Udons in the anime).
His daily attire includes a gray parka and sarouel pants. He dons a hood and a black kerchief to cover the lower half of his face when he goes out on missions as the Submarine Ninja.
Saruwatari is arrogant, brazen, and possesses a short fuse, often seen to habitually kick the nearest object around him to unleash his frustrations. He is reckless and thrill-seeking, choosing work as an assassin in order to have a good fight rather than trying to make substantial money. He is noted to have even given up his own weapon to a target of his to face them unarmed for a challenge.
Perhaps his most significant character trait, Saruwatari hates losing. He takes every challenge seriously and puts everything he has into winning with a single-minded focus. When he loses, he is furious, bitter, and distracted until he can finish what he considers an unfinished fight. His emotional nature makes him easy to read. He is extremely straightforward when it comes to expressing his opinions and desires, and likewise he has a particular inability to lie. Despite this, he refuses to acknowledge his own shortcomings when they are pointed out to him, such as his poor aim or his inability to swim.
He is not a social person. While Nitta and Nguyen consider him to be a friend to some extent, Saruwatari primarily interacts with them either out of necessity or convenience for work-related reasons. In all other regards, he tends to not associate with other people and prefers to be a loner, keeping himself at a distance, and views himself as a solo player even when part of a team.
Saruwatari was raised in the city of Kokura up until high school. He had been taught how to play baseball by his grandfather since he was in elementary school and grew to love the sport. He joined his high school's baseball club where he met his future business partner Naoya Nitta. In their first match, Saruwatari attempted to win with his skill alone, but naturally, due to the difference between his skill and the teamwork of their opponents, his team lost. In the locker rooms, he was approached by Nitta, who managed to calm him down. After that game, Saruwatari and Nitta worked closely together to carry their team's weight until an incident in which Saruwatari lost his temper at Nitta and punched him. At his grandfather's recommendation, Saruwatari transferred to a veteran high school in Yokohama shortly after.
Some time after high school, he strayed from a normal life and was hired by Murder, Inc. He spent the next seven years living in the Tokyo area and became the company's ace hitman. One New Year's Eve after Saruwatari returns from completing an assignment, he sits down to chat with a co-worker, Nguyen, who mentions the infamous Niwaka Samurai in the Hakata area. Intrigued by the urban legend and tired of easy jobs, Saruwatari becomes determined to quit Murder, Inc. and head back to Kyushu to encounter the legendary hitman and other formidable opponents like him. It takes him another half year before he is able to formally leave the company.
Niwaka Samurai Imposter Arc
Upon quitting Murder, Inc. and arriving in Kitakyushu, Saruwatari requests Nguyen give him a list of hitman mediators in the local area. He approaches each and every name on the list but is ultimately turned away for being a hitman with no name. Frustrated, Saruwatari strolls through the city, unsure what to do, when he is called out to by Naoya Nitta. When they sit down for a meal to chat, Nitta confronts him on his hitman business and proposes helping him get work. He believes Saruwatari could become the strongest hitman in Kitakyushu, but he needs a name for himself. A few days later, Saruwatari meets with Nitta in Lady Madonna, where Nitta provides him with ninja garments and weapons before also handing him a list of hitmen in the city, instructing him to kill a few to grab attention. Saruwatari decides to eliminate all the men on the list, but one escapes. The escapee spreads the rumor of the killer ninja, and his fame in the underground begins to grow. Nitta calls him to check up on his progress and lets him know the Kakyuu Association is looking to hire him.
Saruwatari takes the train to Hakata to meet with the right-hand man of the Kakyuu Association, Ruixi Li. In the VIP room of Club Eve, he meets Two-Hand Gun Ricky and the legendary G.G., neither of whom he as ever heard of. When Ricky questions his credentials, he knocks the other man out and is denied a fight between himself and G.G. The two killers are tasked by Li with tracking down and killing the Niwaka Samurai, to Saruwatari's delight. Before he is let go, Li's henchmen put an ankle bracelet on him to track his whereabouts, and he heads back to Kokura to meet up with Nitta. They check the website Undergroundjobs.com, looking for information, and find a post by an apparent fake. Though certain it is an imposter, Saruwatari contacts them to confirm it. He returns to Hakata, and once he meets up with the fake Niwaka Samurai and his 'agent' and learns that they know the real Niwaka Samurai's mediator, Saruwatari threatens them to summon the Niwaka Samurai, holding Yamamoto hostage until Abe has talked with the mediator and established the deal. Once Abe returns, he lets the two men go and waits for the Niwaka Samurai's arrival. A couple hours later, Xianming Lin, disguised as the Niwaka Samurai, appears. Saruwatari challenges him to a death match and lunges at him. They exchange blows for a few minutes, but ultimately Saruwatari gains the upper hand. Just as he is about to kill Lin, Abe and Yamamoto interrupt the scene, driving their van straight toward them, and they snatch Lin before Saruwatari can recover.
After losing his enemy, Saruwatari waits for Nitta to arrive in Nakasu and asks him to find them with the tracking device he put on their car earlier. Once they locate the white van, the two head to the rundown apartment building the signal is coming from, only to find the (disguised) body of Xianming Lin. They confront Abe and Yamamoto, who admit Lin was not the real Niwaka Samurai and informs them they had hired a torturer to get the information out of him, but the fake had died. Saruwatari is furious at the turn of events and is at a loss for what to do when Nitta suggests they take a photo of the body and send it to the Niwaka Samurai's mediator to get the man to come to them. Saruwatari follows through with Nitta's plan and waits for the Niwaka Samurai to appear, taking the body with him. The true Niwaka Samurai shows in the early hours of the morning and demands Lin's body. Saruwatari obliges and is miffed when Zenji Banba's behavior changes upon seeing his friend's corpse. The real Niwaka Samurai is suddenly reluctant to fight, and Saruwatari has to goad Banba into fighting him. They fight only momentarily, however, as Li and his men interrupt them and restrain the Niwaka Samurai. Li rewards Saruwatari with the promised money and the removal of his anklet for helping capture the Niwaka Samurai, and he returns to headquarters with the killer of killers. Saruwatari, furious, ignores the payment offered to him and threatens the two Kakyuu subordinates left behind, demanding they tell him where the Niwaka Samurai was taken.
Saruwatari gets into full gear and infiltrates the Kakyuu Association's office single-handedly, freeing the Niwaka Samurai and assisting him in his escape. The two men work together to face off against the Kakyuu Group's men, retreating to the rooftop, where they make their stand. They cut down more than twenty people together before the remaining men withdraw. The Niwaka Samurai attempts to leave, but Saruwatari stops him. The two fight, and during their battle, they are both unmasked. Once Saruwatari is down to his last shuriken he goes for the finishing blow, only for the Niwaka Samurai to strike the shuriken back at him like hitting a baseball. Saruwatari stumbles to try and avoid the attack but missteps and nearly falls off the rooftop into the crowds of festival-goers below. The Niwaka Samurai saves him and offers a draw and a raincheck on their fight, which Saruwatari begrudgingly accepts.
A few days later, Saruwatari has joined a baseball team in Kokura, and they play against the Hakata Tonkotsu Ramens. He and one of the Ramens' batters begin to argue after Saruwatari nearly hits him in the head with a poor pitch. The two hit each other in the face, causing their hats to fly off, and they recognize each other.
Shou Wang Arc
A week after his fight with Zenji Banba, Saruwatari is still sullenly musing over their encounter when he spots a batting center. He orders Nitta to pull over so he can practice his pitching, leaving Nitta to meet with Tony Lau alone. In the batting center, he finds Banba practicing his batting and invites him to fight. Frustrated at being brushed off, he comments on Banba's batting, causing Banba to comment on his poor pitching.
To complete his employment examination with the Shou Wang, Nitta drives Saruwatari to Byeong-Hee Kim's apartment. Saruwatari finds the man in his parking garage, kills him, and leaves a shuriken next to his body. Nitta drives him to Takashi Unoyama's office in Hakozaki next, where Saruwatari is surprised to find the half dozen mafia members dead inside. Unoyama's lawyer, Tatsurou Myouken, enters the office and runs away in fright when he sees the bodies. Because he had seen Saruwatari's face, the killer throws a shuriken after him as an afterthought and misses. He observes the scene for clues and departs before the police arrive.
Saruwatari is sent to kill the Kakyuu executive Kubota next, but by the time he arrives, all remaining executives have been moved into hiding and a killer from Murder, Inc. is waiting for him. Saruwatari easily overpowers and kills him. Still frustrated by his encounter with Banba at the batting center, he returns to Lady Madonna to practice his shuriken throwing. The next day, he is waiting in a hotel in Hakata on Nitta's orders when he receives a call for help and rushes to the Shou Wang warehouse on Hakata Pier. He finds Zhao standing over the bodies and attacks him, but while his attention is distracted by a light from the water, he slips in the blood. Regaining his balance, he notices Suzuki holding a gun with just enough time to throw himself into the sea.
Banba pulls Saruwatari from the sea and offers to exchange information. Frustrated, Saruwatari declines, and Banba pushes him back into the water. Saruwatari pulls Banba over the side of the fishing boat. After the two give up fighting, they climb into the boat and exchange information on their common enemy.
Saruwatari is wringing out his shirt on the pier when Nitta and Tony Lau arrive at the warehouse. One of the missing Shou Wang members arrives shortly after, carrying his comrade's head in a box, and after taking a look, Tony orders Saruwatari not to fight the Kakyuu Group.
At lunch the next day, Nitta doses Saruwatari's cola with a sleeping drug. When he wakes up, he is bound to a chair in the now-clean Shou Wang warehouse, and Tony Lau is pointing a gun at him. With conditions of surrender dependent on exchanging killers with the Kakyuu Group, the Shou Wang intends to kill their assassin to prevent Saruwatari from switching sides and working against them after the trade. Saruwatari is shocked and infuriated at being betrayed by Nitta, and when Nitta shoots Tony and releases him, Saruwatari punches his friend in the face. He kills Tony with a kunai and checks the money he had paid Nitta for Saruwatari's life, allowing Nitta only a small fraction of the sum. With Tony out of the way, Saruwatari is no longer taking orders from the Shou Wang, and he pursues Zhao with murderous intent.
When Saruwatari arrives at the front gates of the Kakyuu Association's headquarters, he rings the bell and kills the first subordinate to meet him. Several dozen armed men appear at the gate moments later, and Saruwatari wipes them out before heading inside.
In his pursuit of Zhao, he stumbles upon Banba and Lin. Banba confronts Saruwatari and counters his attacks in a hurry as his wound reopens. Saruwatari pitches a shuriken in a curving arc, and it sticks in Banba's leg, causing him to collapse and leaving him unable to fight further. Satisfied with his victorious rematch, Saruwatari thanks Banba for the fight and leaves for Kokura.
A couple of weeks later, Nitta presents Saruwatari with a job targeting a hitman in Fukuoka, and he is furious to soon discover that it is not Banba. He receives details of the job from his client, Sanjou, who expects Méqué to target his organization again soon. Saruwatari's task is to act as bodyguard while Sanjou's subordinates transport cargo from their warehouse to the port.
He meets Yuuta Kase at the Mutagawa Group's warehouse that evening as instructed, immediately taking a disliking to the underling. When Banba, Lin, and Jiro Tanaka force them to stop the truck, Saruwatari faces off with Banba, refusing to help Kase defend the truck's cargo and instead knocking the helpless, livid underling out himself. He allows Banba enough time to call Hironobu Shigematsu to rescue the trafficked children in the truck before their fight, and with the police on the way, the two face off. When Saruwatari pitches a sinking shuriken at his leg as he had in their previous match, Banba hits it back like a baseball, scratching Saruwatari across the face. With the police closing in, Saruwatari concedes the fight and the two quickly depart, leaving Kase unconscious at the scene to take the fall for the dog cages containing children in the back of the truck.
- Saruwatari could potentially be inspired in both name and signature by the professional baseball player Shunsuke Watanabe, who is known as Mr. Submarine in Japan for his record-low submarine pitches.
- He is a fan of the Yokohama DeNA BayStars professional baseball team.
- He cannot swim, and it is noted by Nitta that he had to use a kicking board in swim class to stay afloat.
- He has a low alcohol tolerance, stated in the second light novel. His drink of choice is cola.
- In the manga, he can be seen in the first volume in the background of the Murder, Inc. Tokyo office.
- In the original light novel and manga, he quits Murder, Inc. and heads to Kyushu after hearing about the Niwaka Samurai and becoming increasingly dissatisfied with the work the company gives him, but in the anime he leaves Murder, Inc. solely for the latter reason and does not learn of the Niwaka Samurai until after his arrival in Kokura.