- Seiji is described as having thin, slender eyes and long and high nose. He has long, grey hair and the features of a model - Souhei notes he could mistaken for a woman from a distance. He is fairly tall and slender, and often wears a black button-down shirt and skinny jeans. In terms of ear piercings, on his left ear he has a helix cuff and a tragus stud.
- Former lead singer of a rock band that Haruka liked called <<clouds>>. He recently disappeared to the Invisible City, where he works as a purveyor with Riina. Whereas Riina collects the items, Seiji accumulates a list of what's needed and distributes them among people living in the city.
- Though he is not the leader of the Invisible City, he holds a strong presence among both young and old people in the city. Just at the sound of his voice, people are aware of his authoritative presence.
- He also knows a lot about the city, such as it's history and the people that live there.