Seishirō Hīragi is a member of the Hīragi Family where he was deferential to his father, Tenri Hīragi, yet acted of his own accord unconnected to his brother Kureto Hīragi. While Seishirō felt that his sister, Mahiru Hīragi thought she was better for being their father's favorite, he was outright hostile to his adopted brother, Shinya. Having his aggression around others and contempt for them too indulged in First Shibuya High School, Seishirō was often seen with Yumi, and was particularly violent towards Guren Ichinose and those who associated with him like Sayuri Hanayori. When Seishirō would assume leadership duties in the Japanese Imperial Demon Army on account of his family name, he lost none of his belittling attitude. Seishirō interacts with various other characters throughout the Seraph of the End series, which affects himself and the story. This article details Seishirō's relationships with other characters.
His father and the head of the Hīragi family. He appears fearful to be informed that Guren has secret orders from Tenri, and comments if he really does have orders from his father. With the attacks carried out by the Hyakuya Sect, Seishirō hears Tenri believes there is a traitor in the family yet his innocence has already been proven. Seishirō feels of course it has, he has got the elite Hiragi blood running through his veins. He is loyal to him enough to threaten reporting Kureto's actions to him.
His brother, Kureto is the next head of the Hīragi family. At school Seishirō claimed that he did whatever the hell he wants to do, and that Kureto has got nothing to do with him. Kureto threatens him when he is about to report his betrayal, and with Aoi reinforcing him at the time, Seishirō is powerless to do anything but adhere to Kureto's orders.
They are siblings yet in the same school year, so share different mothers. When Seishirō is tormenting Sayuri, he finds his sword blocked by Mahiru's. He comments that just because Mahiru is father's favorite she acts so high and mighty.
Prevented from punching Shigure due to his arm being taken by Shinya, he questions who asked for the opinion of someone who is but an adopted son. While stating there is nothing to say, Seishirō punched Shinya in the face. With Shinya bearing it, Seishirō says that was a good call. After all, someone like Shinya cannot win against him. Seishirō added that because Shinya made that call, he was chosen by father to be Mahiru's partner, and he should be sure to keep that in mind. With a possible traitor to the family discussed, Seishirō right away asks if it is Shinya, then describes him as pretty weird, and adopted.
Seishiro would see that Kureto's aide, Aoi has killed his guards.
At school he kicks Guren away upon coming across him in a corridor, then claims his foot slipped. Due to face him in the selection exam, but finding our Guren has withdrawn, Seishirō questions what he said just now. His woman has been beaten until she is all unrecognisable, and all Guren wants to do is flee. Seishirō also adds no wonder the Ichinose are all trash. His life is likely saved by Guren after being kicked away by him out of the path of a chain from Saitō. Waiting for him outside of school, Seishirō addresses Guren as Ichinose rat. Just because Kureto thinks highly of him, having made Guren his subordinate, Seishirō asks if Guren thinks he has nothing to fear. Swearing at him, Seishirō orders Guren to not dare ignore him. Angered, Seishirō adds to not mess with him, he will make Guren learn his place, however his punch is answered with a spell tag across his neck. Lied to that Kureto is the suspect, where he purportedly wanted to get rid of Mahiru as a candidate, Seishirō asks why Guren is telling him this. Accepting the situation, Seishirō says to him he sees that Tenri regards Guren quite highly to trust him with such information. Told further that Tenri does not fully trust Kureto, with his future mentioned, Seishirō assumes Guren is trying to suck up to him. If that is the case, Seishirō thanks Guren for telling him. As Guren speaks if anyone else finds out, Tenri will, Seishirō interrupts saying not to worry about it. Seishirō reviews the Guren becoming Kureto's subordinate matter. He sees it as Tenri ordered Kureto to control the Ichinose rat, but really Guren is monitoring Kureto. Getting it, Seishirō questions what should he do. With Guren not wanting Kureto to find out they are involved, Seishirō replies sure thing, he will give him a good punch. After doing so, Seishirō proclaims loudly for all to hear that the Ichinose rat had better remember this.
He disparages Guren in a meeting.
After almost punched by Shigure when he kicked Guren, he announced that the Ichinoses have to be struck to break them like the animals they are. Moving to punch Shigure, he said she was slow.
At the selection exams Seishirō was facing Sayuri Hanayori and said he would strip her as the crowd requested. Calling her an idiot, Seishirō hit Sayuri. With her chest showing, Seishirō addressed her as a lowly live-stock and told her to not show off those filthy things. Beating her some more, Seishirō then went to draw his sword and held Sayuri as he taunted Guren. He brought the sword down on Sayuri and spoke for her to die.
With Yumi defending him from Shigure, he tells her it is fine, she needs to speak straightforwardly to the idiots of the Ichinose. He is seen accompanied by her again.
With Saitō attacking the grounds of First Shibuya High he expresses outrage at his open attack on the Hīragi family. Seishirō runs towards Saitō, and jumping away from his chains laughs that he dares attack this place with such weak weapons. He was likely not aware he was a second progenitor vampire.