Inupi is of average height and a slim build. But, he is commonly spotted dressed in uniform with high heels being a common choice of footwear, making him appear taller. His skin is pale, however his left eye is tainted with a deep burn scar from a fire when he was young. His middle parted sunflower blonde hair with an undercut contrasts his dark emerald eyes and neutral expressional face.
Inupi is usually a stoic and quiet individual. However, he isn't afraid to voice his thoughts and opinions even if it hurts him in the end. He is very committed to the Black Dragon and loyal to Kokonoi, whom he considers his best friend. When he finds people he trusts, he devotes himself to them. He is brave, but will be willing to show his vulnerable side and open up.
As the former tenth-generation leader of the Black Dragon, Inui had control over a massive army of strong delinquents. Later, as he becomes Taiju's assistant, he still has authority over the gang's members, though to a lesser extent.
Inui was unrivaled in a fight before meeting Taiju. Thus, he is a strong fighter in his own right.
As an ardent follower of Shinichiro, Inui inherited an abundance of mechanical knowledge from the bike expert.
- The name Seishu means "blue, green" (青) (sei) and "religion, sect, denomination, main point, origin, essence" (宗) (shu).
- Seishu's surname Inui means "dried, cured, drought, dry, desiccate, drink up, heaven, emperor" (乾).
- His favorite motorbike is the Yamaha RZ350.