Seiji is a teenage boy with short, spiky brown hair and brown eyes. His usual attire is a gray shirt and black pants.
Seiji's most notable trait is his obsession with the head of Celty Sturluson, which he had fallen in love with as a child. This obsession drives him for most of his early life as he wished for nothing more than to run away with it, intent on finding a way to wake it up somehow. Many of the actions he takes in life are meant as a way to make himself more presentable to "her" for when it finally wakes up. After beginning a relationship with Mika Harima, he has relaxed his search for Celty's head, although it still remains at the forefront of his mind. Despite his obsessive personality, Seiji actually possesses a strong moral center. He takes a very chivalrous stance towards women, as shown by his aiding Mika and Anri when they were being harassed. He believes in being honest towards those one cares for and is a firm believer in the power of love. He also shows guilt over hurting others unnecessarily, as noted by his remorse over almost stabbing Mikado during the first Dollars meeting.
As far as abilities go, Seiji possesses little noteworthy. He is shown to be fairly perceptive, being able to discern Mikado's strange behavior after the Toramaru incident despite not being very close to him, and is also decent at improvising in tough situations, as shown how he (unsuccessfully) attempted to fight Shizuo Heiwajima using nothing but a handful of pens in order to defend Mika. He is also good in a fight, as he has been shown to knock normal people out in one punch when he feels threatened.
Celty has noted that Seiji is calm and level-headed most of the time, such as when she chose him to explain why he and Mika- among many others- were in her home.
As a child, Seiji and his older sister Namie Yagiri were left in the care of their uncle Seitarou Yagiri, due to their parents' busy schedules. However, as their uncle was also a frequent traveler, the siblings spent most of their time playing together on their own. As the two spent more time together, Namie eventually developed a very possessive, incestuous love for her brother which Seiji never reciprocated. One day, while they were playing, Namie took Seiji to their uncle's study to take a look at some of the more curious items in his collection, in particular, a disembodied head kept in a glass jar. Seiji immediately developed an obsession with the head and would frequently ask Namie to take him to see it. As the years went on, Seiji fully admitted to falling in love with it, much to his sister's horror.
As life went on for the two of them, Namie became the head of the Yagiri pharmaceutical company and took possession of the head in order to research it. Seiji, in turn, took a job at their main facility in an attempt to get access to it. As a member of the family, Seiji had free access to the facility, including the lab where they ran tests on the head. He memorized the guard's patrols and schedules and eventually stole the head as soon as an opening presented itself.
Seiji kept the head in his apartment and attempted to live his life as normal, hoping the company would give up their search. One day, he came across Anri Sonohara and Mika Harima being harassed by a small group of thugs. Disgusted by the thugs' vulgar demeanor, Seiji stepped in and chased them off. After making sure the girls were unharmed, he continued on his way home, however, his act of heroism unwittingly earned the affection of Mika, who began stalking him from that day onward.
The stalking would eventually reach the point to where Mika would repeatedly attempt to break into Seiji's home, prompting him to improve his security for his apartment. One day, while Seiji was out, Mika manages to successfully break through the lock and discovers the head. Seiji returns and finds her staring at it. As Mika tries to explain herself, Seiji grabs her by the head and slams it repeatedly against the wall until she falls unconscious. Thinking he had killed her, Seiji calls Namie to help clean up the evidence, handing the head back to her in the process. Unbeknownst to him, Mika had survived and agreed with Namie to undergo plastic surgery to look like the head in order to obtain Seiji's affection. After the surgery is completed, Mika breaks out of the facility before they could administer memory altering drugs. Seiji catches her riding on the side of one of the company vans and, thinking his sister had somehow managed to attach the head to a human body, proceeds to try to locate her after he loses track of the van.
Dollars/Mika Harima ArcEdit
Seiji starts school at Raira Academy along with Mikado Ryuugamine, Masaomi Kida, and Anri Sonohara. He is elected to the Health Committee on the second day of school. In the anime, he is introduced when he arrives at school late and notifies Shojiro Kitakoma that he would not be attending school for the remainder of the year. Seiji would spend the initial half of the arc off screen, searching for the girl with the scar on her neck using any method he can. Miraculously, he locates her on the street and the two begin dating not long after. After Namie finds out the two of them made contact, she has several of her men keep tabs on them as they spend time together. Mika, wanting to get out of Namie's watchful eye, makes several attempts to break her hold on Seiji, tossing his cellphone into a fountain to keep it from being tracked as well as trying to convince him to leave the city with her.
One day, as the two spend time in a park, Mika once again tries to convince Seiji to leave with her. Their argument catches the attention of Celty Sturluson and Shizuo Heiwajima who were conversing nearby. Celty recognizes Mika's appearance and gives chase, prompting her to run away. As Shizuo attempts to help, Seiji intervenes by stabbing Shizuo with several pens he had kept in his pocket. Shizuo questions Seiji on his devotion to Mika to which Seiji replies that, despite not knowing a single thing about her, he still loves her unconditionally. Frustrated by his irresponsible attitude, Shizuo headbutts Seiji and tosses him into a nearby truck. After Shizuo leaves, Seiji attempts to follow Mika but witnesses her going into a subway with Mikado. Moments later, the police take him in for questioning.
After being released from custody, Namie has Seiji brought to her, assuring him that she would locate the girl for him and that Seiji would have to stay at the facility for a while. As he begins to grow anxious about finding Mika, Seiji threatens one of Namie's employees with a knife to tell him about what she had planned. After finding out about Namie's meeting with Mikado, Seiji makes his way towards the Dollars meeting place. While the crowd erupts into chaos over Celty's revelation, Seiji confronts Mikado over Mika's location. In a fit of desperation, Seiji attempts to stab Mikado but is blocked by Celty. Before the fight can continue, Mika runs out and stops Celty's scythe, explaining her actions up until that point and revealing to Seiji that she was not the head he had spent so long trying to obtain. The revelation that he had been chasing an imposter all this time causes Seiji to go into shock. After Mikado offers some words of comfort, Mika eventually takes him home. After things settle down, Seiji and Mika return to school and he agrees to accept Mika's love until he can locate the real head as Namie had fled with it and went into hiding.
Yellow Scarves ArcEdit
Seiji makes a few sporadic appearances during this arc but his main scene involves him giving words of encouragement to Mikado. He tells the young boy that, if he truly loves Anri, he will accept her for what she is, regardless of what he learns about her.
Seiji is not seen until the end of the arc as one of several people who are invited to Shinra and Celty's hotpot party. He asks Celty if she is still looking for her head. Celty replies that, for the time being, she has put the search on hold. Seiji is disappointed but declares that he will simply have to look for it himself.
Daily Life ArcEdit
Namie had hired the Orihara Twins to spy on Seiji and Mika to assess the state of their relationship. After seeing they had become too close for comfort, Namie lures Mika away during a date in order to scar her face irreparably so that Seiji would reject her. Concerned about Mika's sudden departure, Seiji goes through his contact list to try to locate her, eventually getting the location from Shinra. Seiji stops Namie just as she was about to go through with her plan, scolding her about her actions. Seiji then kisses Namie in an attempt to show her randomly kissing someone out of the blue is wrong, however, Namie takes it the wrong way and leaves in embarrassment. Seiji takes Mika home and reveals that when he kissed Namie, he had placed his finger between their lips so that it technically didn't count. Namie, however, remains oblivious to this fact.
Kadota's Coma Arc Edit
After Mika learns that the Nebula might be after her and Seiji, they go to Celty's apartment. After seeing Celty's head on the news, he decides to break into the police station, before Shingen stopped him.
Final Curtain ArcEdit
At the end of the volume, Seiji and Mika apply to study abroad in America, only to follow Celty's head to Chicago (taken by Nebula and his sister Namie for research).
- Main article: Seiji and Mika
- Main article: Namie Yagiri
To call Seiji's relationship with his sister strange would be an understatement. Namie bears a strong incestuous love for Seiji, born out of years of isolation and as such, makes frequent advances toward him when she gets the chance. However, Seiji simply cares for Namie as a normal sibling and greatly wishes for her to give up her criminal lifestyle for her own safety. That being said, he is not above using her if it means he will obtain Celty's head somehow.
- In the anime, Seiji shows up to the first day of school to say that he will not be coming to school. In the light novel, he does attend school and is the Health Committee representative in his, Mikado, and Anri's class.