Age: 15 (close to 16)
Birthday: May 8th
Height: 165 cm
Weight: 58 kg
Mother: Makiko Nagi
Father: Kago Sohaku
Soichiro is a brash, blonde, spiky haired teenager, who enjoys fighting. As a child, Soichiro was bullied and tormented because his family consists of demon exorcists, and were not well liked as a result. The apartment he shared with his mom was also covered in offensive messages. Soichiro started fighting early as a result, beating up those who tormented him. In elementary school, he met up with his best friend Bob Makihara, and they started calling themselves the Knuckle Bombs. The pair go to school beating up everyone they find until they have been proven the strongest. Upon arriving at Todo Academy, Soichiro quickly learns he is nowhere near as strong as he thought.
Soichiro fights using street fighting methods as opposed to real martial arts. He prides himself on being a thug and therefore he fights without honor, usually with cheap shots, etc... However, he does seem to draw line when it comes to hitting women. After training with Maya Natsume, he learns more martial arts moves including the Forged Needle attack. Throughout the Manga, it is shown that Soichiro and Aya Natsume have some sort of mental connection between them that calls to the other when they are in trouble.
Soichiro is a teen obsessed with fighting and takes a lot of pride in that. Ironically, he believes that fighting doesn't resolve anything, and fights more for fun than anything else. Initially, he is believed to have an average, or below average intelligence. He is very cocky and gets angered easily, and always has wise cracks. He isn't very good with being around girls, because he had little contact with them up until high school because they thought he was creepy. When he enrolled into Todo Academy, he fought Masataka Takayanagi resulting in him losing his first major fight. He falls in love with Maya Natsume, because she is similar to him. Through out the series, he repeatedly has shown he has trouble showing any affection towards people he cares about. Soichiro is slowly changing, and starting to become more intelligent and starting to build the charisma of a leader. For example, Soichiro actually gets Kagesada Sugano (almost solely referred to by his nickname asshat in the manga or scum in the anime) to once again become an active Juken Club member. Their initial encounter seems to inspire Kagesada to quit his typical cowardly behavior and fight. Soichiro also occasionally breaks the fourth wall. The two most notable instances of this are in chapter 3 when he states that he's supposed to be the "hero", and in episode 24 of the anime when he refers to Masataka as a secondary character.
Aya is in love with him, but he seems uninterested because he is in love with Maya, although later on in the series he shows signs of liking or possibly loving her, not reacting so drastically when Aya shows her affections and trying to protect or comfort her (such as in chapter 109 when Soichiro, after Aya's fight with Hirohiko, goes to check on her because he is worried about her even though he won't admit it, which ends up in him receiving a kiss from her outside of the shower where they first met). Maya's feelings for Soichiro are unknown. Later on, Shizuru Kamura, she also develops a crush on Soichiro and views Maya as an enemy as a result. Soichiro also seems to be rather protective of his mother, even though it doesn't seem so initially. He plans to drop out and take over her work so she can finally retire after he competes in this years tournament.