Seiji Yagiri (矢霧 誠二,Yagiri Seiji) is a classmate of Mikado's . He quits school claiming he has something more important to do with his life, and later says that his goal is to find his 'true love' and run away with her. He is actually in love with Celty's head, which he first discovered when his sister, Namie snuck him into their uncle's study to look at the head. Seiji was being stalked by Mika Harima, until she broke into his apartment and discovered Celty's head, which he stole from his sister's research lab. Seiji then tried to kill her, and had his sister transplant Celty's head onto Mika's body. However, Seiji then finds out that he hadn't killed Mika, and that she had only gotten cosmetic surgery to look like Celty's head.
Dollars/ Mika Harima ArcEdit
At first Seiji was being stalked by Mika Harima. When Seiji got home she busted in,and Seiji whacked her in the head with a bat so she wouldn't tell anyone about the head. Then he called his sister saying that he killed someone. She came over and took the head back with her,and the "dead" Mika.
The next day Seiji attends school,and gets questioned by Anri Shonohara when Anri was looking for Mika. Seiji brushed her off. Then later he found the head on a human's body. The next day, she ran off because she was worried about Celty finding her. But then Seiji saw Celty and Shizuo near the girl so he ran at Shizuo and stabbed his leg with a pen.
Shizuo got angry so he picked Seiji up,and asked him questions. Seiji said he didn't know so Shizuo got pissed and threw him at a semi. Seiji then was picked up again. Seiji yelled at Shizuo that he is in love with the girl,and stabs Shizuo in the hand with a pen. Shizuo head butted Seiji and walked away. That night Namie figures out. The next night Seiji spots Mikado after the Dollars meetig so Seiji gets out a knife and runs at Mikado yelling "Where is she!"
Celty gotin the the way,and then they started fighting,but the girl with a scar on her neck yelled stop,and told the truth about her being Mika Harima. Seiji gets in shock,and Izaya Orihara teases him about it. But then they date even though Seiji is still wanting the head.
He is with Mika when she warns Anri about Takashi Nasujima.
Seiji isn't seen until the end of thr arc as one of several people who were invited to Shinra and Celty's hotpot party. He asks Celty if she was still looking for her head. Celty replies that, for the time being, she has put the search on hold. Seiji is disappointed but then declares that he will simply have to look for it himself.