Souichiro 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 Souichiro and Aya Natsume have some sort of mental connection between them that calls to the other when they are in trouble.
Souichiro 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. Souichiro is slowly changing, and starting to become more intelligent and starting to build the charisma of a leader. For example, Souichiro 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. Souichiro 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 refferes 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 Souichiro, 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 Souichiro are unknown. Later on, Shizuru Kamura also develops a crush on Souichiro and views Maya as an enemy as a result. Souichiro 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.
The Magabarai blood that runs through Souichiro's veins allows him to tune into the Red Wing Resonance of anyone of the Red Wing blood lineage. Souichiro, when using his family’s power can transform, turning his hair black (as well as making his hair no longer spike upward) and his fists black as ki enters into him and exits through his dragon gates, activating his chakra thusly. In this form, the limitless amount of ki flowing through him greatly strengthens his supernatural abilities. However, Souichiro was inexperienced with this phenomenon and the resulting surge of ki forced tremendous strain of his body, and in some cases, was on the verge of being consumed by his own dragon; with the possibility of going mad or dying. Only while in Sohaku's custody, was he able to freely access his chakra, without the assistance of the Red Wing Resonance Effect. According to his mother, if the Nagi spirit is disturbed then they can’t use this power. Recently in the Takayangi mansion, Souichiro does battle with Kabane in a different and theoretically more powerful armor and has limits released as well but Souichiro, using his Demon Exorcist Form, easily breaks through his armor, showing that his powers have greatly increased beyond that of Kabane's.
It should also be noted that Souichirou has consumed an unknown amount of ki and abilities, since the full list of the people he was forced to devour was never shown.
Souichiro claims he will never lose again after that, but his actions have caught the attention of the Executive Council. An enforcer named Tsutomu Ryuuzaki comes to handle the two new students, Souichiro and Bob. Ryuuzaki torches Bob's motorcycle, and goes after Bob's girlfriend Chiaki (in the anime, he merely assaults Chiaki; in the manga, he actually rapes her). Bob and Souichiro show up and are both beaten. The two decide to join the Juken Club, that Maya, Masataka, and Aya are members of in hopes they can get stronger and get revenge. Souchiro manages to improve at an astounding rate, performing techniques that should take years to master in a matter of days.
Not long afterward the Executive Council attacks the Juken Club at a bowling alley. Souichiro fights Kōji Sagara(Saga Mask), and defeats him. He later encounters Mitsuomi Takayanagi, the Executive Council president; he somehow manages to keep getting up despite being hit with the Takayanagi family secret technique, but the fight is interrupted by Bunshichi Tawara. Following the incident, Bob and Souichiro find out the history behind the Juken Club, Maya, and Mitsuomi.
A rivalry is set up between Mitsuomi and Souichiro, as they both seem to love Maya. In the process of preparing to fight Mitsuomi at the schools tournament, Souichiro awakens his demon exorcist-self, his hair is no longer spiked up and turns black. Souichiro has an unlimited amount of ki in this form. Neither the anime nor the OVAs have proceeded farther than this.
The manga has proceed much farther than the anime or OVAs. After awakening his demon exorcist-self, Souichiro learns the Takayanagi family is traditionally supported by the 12 founding families as represented on the family's crest, the Rising Phoenix. The families are further split into Red Feathers and White Feathers. The setup was disturbed by Mitsuomi's takeover, a new alliance has formed called 'F'.
The first of these opponents is Tessen Ishiyumi. At the end of this fight, Souichiro "loses" his left leg to his own dragon, but steals Tessen's powers. Afterward Souichiro meets Shinichirō Kurei, the Juken Club's vice president, and learns there is another member of the group (although he is inactive). Souichiro goes to meet the member, Kagesada Sugano (asshat), and after a conflict he convinces Kagesada to become an active member again.
Later he encounters Mataza Tsumuji, on Mataza's quest to find a computer chip hidden with the Enmi Family (Enmi is one of the 36 branch families) Scroll. He defeats Mataza, but loses control of his dragon thanks to Mataza tainting him with his hatred. His mother, Makiko Nagi sucks away all his power saving him, but loses her only remaining arm in the process.
Tessen returns looking to reclaim his powers that Souichiro stole from him. As they are about to kill each other Maya and Mitsuomi stop the two of them. Afterwards, they all head out to take on Souichiro's father together. Even though he leaves himself open to attack Souichiro can't bring himself to kill his father. The fight is short-lived, however as Mitsuomi quickly beheads Sohaku. Through Aya's flashbacks to ancient times we learn Sohaku has power over death, and has the ability to store his consciousness in his right eye. Later Noriko Shindayuu Inue is shown poking out Souichiro's right eye, and it is implied that Sohaku's eye is implanted.
During training, Maya asks each of the members what they fight for which causes Souichiro to examine what has happened to him up to that point. He confesses to Aya that his mother was still doing the work of the Demon Exorcist and that he would like to take up her mission so she could finally quit. He admits that the tournament will most likely be his last act as a student. After leaving a saddened Aya, Souichiro runs into Saga Mask, who has come for their rematch. Souichiro tells him that he will die if they fight but Saga Mask proceeds to say this is the next step in his goal of reaching the pros. However, before he can finish, Souichiro moves up to him and strikes him in the stomach in an instant. The strike is so powerful that it explodes Saga Mask's entrails out his back all over the ground. In stunned shock, Saga Mask asks a gloomy Souichiro who he is and if he is 'human'. Though this action is most likely an illusion created by Souichiro's intent to kill Saga Mask. Most likely this was done in order to prove the point to Saga Mask that Souichiro was far stronger than he was.
After Masataka wins the first round of the tournament for them, Souichiro is pestered by their next round opponents. They stand on a high ledge and throw rocks at him, greatly annoying him. The others try to plan out what to do, but Souichiro just jumps his way up and hits them with the Forged Iron Needle Attack wiping them all out at once. When Bob asks how he got up there, he comments that he doesn't know.
In the 3rd round Souichiro starts to step up as the Advance Guard but Kagesada (asshat) Steps up and takes that role for himself. Souichiro starts to argue but is stopped by Masataka. Souichiro is shocked to see Kagesada can actually handle it. Once the newly enhanced captain of the Jujutsu Club arrives the tide turns against Kagesada, Souichiro and Bob try to get the judge to stop the fight to no avail. Kagesada insists they not interfere, seemingly inspired by his first encounter with Souichiro. Even when Kagesada's arm is broken by his opponent, Souichiro still has to sit on the sidelines realizing he can do nothing else he merely cheers Kagesada on.
During Aya's conflict with Hirohiko Myouun Kabane and Nokimi Mayutsubo, it's implied that if Souichiro fought another person with special powers (such as a person controlled by Nokimi) he would likely be overtaken by his dragon again. This is what Sohaku is waiting for as he plans to use this as his chance to take over Souichiro's body.
Souichiro actually ends up interrupting Aya's fight with Hirohiko after Kagesada's fight ends in a double KO, wining the round for the Juken Club. Afterward the Juken Club prepares to go home at the end of the day. Masataka hesitantly decides to go with Souichiro to eat given he has nothing better to do, yet refuses to pay. Realizing they need to eat so they will be at their peak for the tournament, yet both of them are broke, Souichiro intimidates a waiter into giving them free food. Souichiro thinks back to their fight back in the lunch room, and thinks that his first real fight in the school was against the strongest person in the school. Given Masataka's modest nature he doesn't get what Souichiro means. Souichiro comments that the intimidation from fighting Masataka gave him goosebumps. He also indicates the person inside his eye (Sohaku) is even more intimidated, as he removes his eyepatch for a second. (Given his claim of memory loss after the beheading of Sohaku, this is actually the first time Souichiro indicates he knows Sohaku is inside him, and that the eye is not his own.) Souichiro even says that he thinks everyone except Masataka uses their power as a "turtle shell" to hide their cowardice. The talk of the past has Masataka thinking back to what Bunshichi told him about not being in the wheel of fate, and says he'll make a place for himself as he stands up. Instead, Souichiro tells Masataka to sit down. Since he is trapped inside the wheel, he thinks Masataka should break it up from the outside and free him. Souichiro then follows Masataka to the Takayanagi Clan residence. He tries to forcibly get in to reach the top floor where Masataka and Mitsuomi are fighting, but he is met by heavy resistance made of Mitsuomi's private army and Hirohiko's younger sister. Hirohiko helps him by having the men stand down. Pretty soon, Hirohiko and Nagi engage in a fight where Souichiro uses his Demon Exorcist form, easily breaking through Hirohiko's first set of armor after being stabbed repeatedly by Hirohiko's blades. Nei lends her body to Hirohiko, forming new armor. During the fight he unleashes all the power he absorbed killing Nei, Hirohiko's younger sister in the process. It is also during this crucial time that Sohaku is making his bid to take over Souichiro's body, earning the full anger of Souichiro's mother whom had felt everything her son was going through during the fight. It appears that Souichiro was aware of Sohaku's ambition to take over his body and had asked Masataka that if he could not win against Sohaku in the mental battle to come, that he would be the one to end his life.