Namie Yagiri's little brother, deeply in love with Celty to the point of obsession.
Seiji's trouble started when he was just a little boy under his sister Namie's care. One day, Namie snuck into her father's study with him to show him the cool head floating in a jar that was sitting on his desk, the head of a Dullahan named Celty. From the moment he first laid eyes on it, as a little kid, he became enchanted with it and continued asking his sister to help sneak back in to see the head.
When he grew older, Seiji continued seeing the head even as his older sister took control of the company and showered him with gifts and affection. At one point, Seiji took the jar with the head in it and kept it in his room. However, a stalker named Mika fell in love with him and saw the head in the jar when she broke into his apartment. Seiji responded by slamming her head into a wall to keep her mouth shut, killing her, then called his sister to deal with it.
Some time after that incident, Seiji showed up on the first day of school to the same class that Mikado, Kida and Anri were attending to announce that he would no longer be attending school, as he believed that his new purpose in life was to spend every waking moment with the girl that had Mika's body and Celty's head. Namie even agreed to support him financially out of her own love for him. This strange attitude confused most of his classmates, but Seiji didn't care as long as he could spend his life with Celty.
As of Episode #12, it turns out that Mika was actually still alive and well after that incident, but Shinra (under Namie's direction) performed surgery on her face to make it look like Celty's head had been attached. Mika went along with it to get closer to Seiji, and Seiji never figured it out until she came out and admitted it. However, he decided to stay with Mika because her head now reminded him of Celty's head.