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. Whenever he finds himself about to get into a fight, he recites several laws that his attacker is about to break as well as the consequences, in an attempt to dissuade the attacker, which is not always effective. 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.
It is revealed at the end of the anime that Kōshi'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. However, sealing his power was a big mistake for her to do. It only makes Kōshi becomes a useless man with weak heart who is unable to use his martial arts. Because of that, he quits learning martial art and often gets bullied. He can not fight or defend himself properly against the assassins who are after his life; thus he has to depend on Momoko. There are clear indications that the seal has been breached during the last episode, but Kōshi never uses his powers; Momoko manages to defeat their ultimate enemy before Koushi can act. Nevertheless, that final enemy was momentarily distracted by the awakening of Kōshi's powers, which may be an indication of their capacity.
Kōshi initially wants nothing to do with Momoko or the martial arts war that he has been thrown right in the middle of. He instead focuses even more on his studies so as to keep his mind off of being targeted for assassination. Gradually, he seems to develop a greater appreciation and caring for Momoko and manages to overcome his tendency to freeze up in the face of violence. Also, at the end of the anime, when Momoko was gone for some days it is shown that Kōshi could not live properly without her; so much that he had delusions of her cheering him on when confronted by an assassin, his house was a mess and he could not tell the time properly. Best explained by Kōshi himself: 'I see. I cannot deny it anymore. My "daily life" cannot be away from her.'
On the Chinese Zodiac, the dog (Kōshi Inuzuka) gets along very well with the horse (which could be Sanae Nakajima), and does not get along well at all with the dragon (which is Momoko Kuzuryu).