- 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 pants. In terms of ear piercings, on his left ear he has a helix cuff and a tragus stud.
- "The Invisible City’s leader who manages its youth. With his androgynous facial features he is charismatic and has calm resolution." 
- Former lead singer of a rock band that Haruka liked called <<clouds>>. Two years ago, he disappeared to the Invisible City after an incident that happened in <<clouds>>. Currently works as a manager in the Invisible City, which includes assigning people jobs and making sure conflict is resolved in the city. Though he denies it, everybody sees him as the Invisible City's leader. 
- He is a skilled guitar player.
- Seiji is not his real name: the members of <<clouds>> all used aliases.