Shunsuke Saruwatari (猿渡俊介, Saruwatari Shunsuke) is a top skilled 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 battery mate Naoya Nitta from high school. 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 wears a gray parka and sarouel pants and dons a black kerchief to partially cover the lower half of his face when he goes out on missions as the Submarine Ninja with his hood up to best hide his identity.
Saruwatari is arrogant and brazen along with a short fuse, often seen to habitually kick the nearest object around him to unleash his frustrations. He is thrill seeking, working as an assassin to have a good fight more 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.
He is not much of a social person. While Nitta and Nguyen consider him to be a friend to at least some extent, Saruwatari primarily interacts with them either out of necessity or convenience for work. In all other regards, he tends to not associate with people and prefers to be a loner, keeping himself at a distance from others. 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 pointed out to him, such as his poor aim or his inability to swim.
Saruwatari was raised in the city of Kokura up until high school. He was 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 differences of skill and teamwork of the opposing team, his team lost. He was approached by Nitta in the locker rooms and was calmed down. Since then, Saruwatari and Nitta worked closely together to carry their team's weight that was until Saruwatari lost his temper at Nitta and punched him, his teammates who tried to stop him, and their coach. After the incident, Saruwatari transferred to a veteran high school in Yokohama under his grandfather's recommendation.
Some time after high school, he strayed from the modest life and got 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 returned from completing an assignment, he sat down and chatted with a co-worker of his, Nguyen, who mentioned the infamous Niwaka Samurai in the Hakata area. Intrigued by the urban legend, Saruwatari became determined to quit Murder Inc. and head back to Kyushu to encounter the legendary hitman or other formidable opponents like him, although it took him another half year before he could formerly leave the company.
Niwaka Samurai Imposter ArcEdit
Upon quitting Murder Inc. and arriving in Kitakyushu, Saruwatari requests Nguyen to 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 to help him get work and to become the strongest hitman in Kitakyushu. 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 organizations attention. However, Saruwatari decides to eliminate all the men on the list but inevitably one escapes, much to his fortune later. Nitta calls him to check up on his progress and lets him know the Kakyu Association was looking into hiring him.
Saruwatari takes the train to the Hakata ward to meet with the right-hand man of the Kakyu Association - Ruixi Li. He is tasked to track down and kill the Niwaka Samurai, much to his delight. Before he is let go, Li's henchmen put an ankle bracelet on him to track his whereabouts. Saruwatari heads back to Kokura and meets up with Nitta to find where they got get a trace on the infamous killer. They check undergroundjobs and find a post by an apparent fake. Although certain it is an imposter, Saruwatari contacts them to confirm it. Once he meets up with the fake Niwaka Samurai and his 'agent' and hears they know the Niwaka Samurai's mediator, Saruwatari threatens the two to summon the Niwaka Samurai. He keeps the imposter - Yamamoto - hostage while his partner Abe leaves to talk with the mediator and establish 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 the man 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 towards them and snatch Lin before Saruwatari could recover.
After losing his prey, Saruwatari returns to Nitta and asks him to find them with the tracking device he put on their car earlier. Once they locate where they are, the two head to the rundown apartment building, only to find the (disguised) body of Xianming Lin. They confront Abe and Yamamoto, who admit Lin was not the real Niwaka Samurai and hired a torturer to get the information out of him but the man had 'died.' Saruwatari is furious at the turn of events and is at a loss of 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 fake Lin's body with him. The Niwaka Samurai shows up after a few hours late into the night and demands for Lin's body. Saruwatari obliges and is miffed when Banba's behavior changed upon seeing his friend's body and the sudden reluctance to fight. Saruwatari goads Banba into fighting him, who finally listens. They fight only momentarily, however, as Ruixi Li and his men interrupt their fight and restrain the Niwaka Samurai. Li awards Saruwatari for helping capture the Niwaka Samurai before heading back to their headquarters with the Niwaka Samurai. Saruwatari, furious, ignores the payment offered to him and threatens the two Kakyu men left behind, demanding to tell him where they took the Niwaka Samurai.
Saruwatari gets into full gear and infiltrates the Kakyu Association's office single-handedly, assisting the Niwaka Samurai in his escape. The two men work together to face off against the Kakyu Association's men, retreating to the rooftop of the building where they make their stand-off. They cut down more than twenty people together before the remaining men withdrew. The Niwaka Samurai attempts to leave, but Saruwatari stops him. The two fight, and 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. Saruwatari stumbles to try and avoid the attack but missteps and narrowly falls off the rooftop when the Niwaka Samurai saves him. The Niwaka Samurai offers a draw and for them to resume their fight another day, which Saruwatari begrudgingly accepts.
A few days later, Saruwatari has joined a baseball team in Kokura along with Nitta and plays against the Hakata Tonkotsu Ramens grass-lot team. He and one of the batters - Zenji Banba - 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 them having a good look at each other and realize their identities.
- 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.
- 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 even learn of the Niwaka Samurai until after his arrival in Kokura.