Kōshi is a handsome, intelligent, yet physically weak seventeen-year-old high school student who is the son of Unken Inuzuka, a martial arts master and member of one of the twelve Zodiac Master families of Japan. His family is associated with the dog of the Chinese zodiac. However, against his father's wishes, he wants nothing to do with martial arts. He has avoided training ever since being terrorized by bullies at a young age, which initially caused him to freeze every time that he sees violence. Due to this, he wants to someday become a prosecutor so that he will be able to punish crimes effectively. As such, he is constantly seen reading books on laws and codes that govern Japan. He seems to believe that martial arts do not have any realistically applicable power and the rule of law can be used as a weapon against enemies.
He is shocked by his father's decision of an arranged marriage and continuously has to fight off the love advances of Momoko.

It is revealed later that Koushi's powers as a martial artist were sealed by his mother, who wanted to protect her only child from the hardship she and her husband had to face as martial artists. She also believed Koushi had a gentle nature which would cause him to regret getting into fights.