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 and likes the challenge as he works 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, but as blunt as he is 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.
- 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 cannot swim and is noted by Nitta that he had to use a kicking board in swim class to stay afloat.
- In the original light novel and manga, he quits Murder Inc. and heads to Kyushu after hearing about the Niwaka Samurai and is 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.