Soichiro Arima (有馬 総一郎 Arima Sōichirō) becomes Yukino Miyazawa's rival at school. Very handsome and popular, Soichiro excels at kendo and becomes a national champion.
After a lifetime of striving to be "perfect", Soichiro's dark side begins to manifest itself by 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 any one but him being close with her. Hideaki told Takefumi that the only reason Hideaki Asaba was allowed to be friends with Yukino was because Soichiro knew that Hideaki would never fall in love with Yukino and Yukino would never fall in love with Hideaki.
Soichiro was born as the son of Reiji Arima and a woman named Ryoko. However, Ryoko was a gold digger who only wanted Reiji for his money (due to him being an Arima family member). When she found out he was an illegitimate child and would therefore get no inheritance from his family, she became highly disappointed and very angry, taking it out on Soichiro throughout his childhood.
For the first three years of his life, Ryoko severely abused her son and blamed him as he was no longer a leverage for her and for not giving her what she wanted. She beat him until he bled, bruised him beyond recignition and labeled him as a monster to other children. She also had starved him until he was very malnourished and only gave him stale bread to eat.
By the time Reiji had found Soichiro, he was nearly on the verge of dying, was ill and completely bruised. Because of that, he was put in the care of his aunt and uncle Shouji and Shizune Arima to get him away from Ryoko. Due to being the son of the Arima family's illegitimate child (Reiji), Soichiro was outcasted by most of the family who regarded his parents as their disgrace.
He is at first smitten by Yukino, unaware that she secretly hated him and constantly tried 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 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 an emotional sanctuary in each other. As with Yukino, he feels more in touch with his true self as he falls in love. But along with love, he is apprehensive of his other emerging emotions, such as becoming increasingly jealous of Yukino's friends, her activities, 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. A When the charade reaches a critical point that 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. After spending 10 days together, both father and son grew fond of each other and reestablished their bond. 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.
- Soichiro's birthday is on April 29 and he is a taurus.
- As a child, he was quick in learning how to read and pronunciate faster than other children his age.
- He received many confessions during his middle school and elementary school years.