Soichiro Arima is the male protagonist of the series, starts out as Yukino's rival at school, and ends up as her boyfriend and eventual husband . A handsome and popular boy, Soichiro excels at kendo and becomes a national champion. He is at first smitten with Yukino, unaware that she secretly hates him and constantly tries to compete with him. After unwittingly discovering that Yukino is not the model student everyone thinks she is, Soichiro blackmails her into doing his school work so that he can spend more time with her. After resolving the issue and becoming each other's first true friend, they eventually fall in love. Yukino learns that Soichiro, like her, is making an effort to "be perfect" for a reason. While Yukino is motivated by praise and vanity, Soichiro is trying to cover his "real" self, a self damaged by childhood trauma that he fears may be destructive. Yukino and Soichiro were able to find emotional sanctuary in each other. While Soichiro comes from a distinguished family of doctors, he was abandoned by his parents, who were the family's disgrace, and raised by his uncle and aunt. Fearing that he may turn "bad" like his parents, he is always trying to hide behind a mask and is constantly fighting his own demons. As with Yukino, he becomes more in touch with his true self as he falls in love with her. However, he is apprehensive of the other feelings what emerge along with his love, such as his increasing jealousy of Yukino's other friendships, her activities and her life without him. When Yukino unknowingly damages Soichiro's bond to her, he finds himself becoming jealous and afraid. So he begins to wear another facade that he hopes will fool her and protect her from his "ugly" self. After a lifetime of striving to be "perfect", Soichiro's dark side begins to manifest itself through vengeful feelings against those that hurt him in his family, and he realizes that he has the ability to be cold and cruel. One aspect is his possessiveness of Yukino, hating for any one but himself to be close with her. Hideaki told Takefumi that the only reason Hideaki was allowed to be friends with Yukino was because Soichiro knew that Hideaki would never fall in love with Yukino and she would never fall in love with him. When the charade reaches such a critical point that it threatens his relationship with Yukino, she steps up to save him by opening him to a true and complete love. After a number of violent outbursts (one of which has him making love to her in the school library rather suddenly, which later has him thinking she wasn't willing, and almost drives him mad) Soichiro finally understands that to love, he must be willing to trust and be trusted, even enduring the suffering that comes out of love. Yukino is able to convince Soichiro that she truly loves him for who he is, and that she is willing to support and suffer for him. From that point on, Soichiro truly and completely opens up to Yukino, his friends, and his beloved adopted parents.
Soichiro's union with Yukino has come full circle, as both are finally free from the oppression of the conflict within themselves. Even though Soichiro cannot forget the past, he no longer allows it to control and hurt himself or anyone else.
After high school, Soichiro marries Yukino and they have three children. Their oldest child is a daughter named Sakura, a beautiful girl who resembles him in looks and talent. Although his original ambition was to follow his family's profession and become a doctor, he follows his childhood dream and becomes a policeman. He eventually becomes a highly decorated police inspector, and is preparing to sit for the exams of a Commissioner's promotion (at the insistence of his superiors) at the age of 30.
Soichiro was also reunited with his father, Reiji Arima, a famous jazz pianist who lives in New York but returned to Japan for a concert. Reiji threatens to harm Yukino, upsetting Soichiro, and he holds a false pistol to his forehead in the center of a park. After spending 10 days together, both father and son grew fond of each other and reestablished their bond. Soichiro spends most of his time assisting his father, and Reiji rewards him by taking him to a nice dinner with Yukino. Unfortunately, he tells Soichiro to go home because he does not feel like a father and would be better off without him. When Soichiro and Yukino are ambushed by Ryoko, wanting to meet with Reiji, he comes to their aid. He leaves for New York once the event is over, but promises to return to Soichiro the following summer. Reconnecting with his father helps Soichiro deal with his problems and heal the wounds of his childhood. In the anime television series, Soichiro Arima is voiced by Chihiro Suzuki in Japanese and Christopher Nicholas in English.