In the manga, Kazuya met Shiori during the time she was at the hospital and the two started dating. In the anime however, he is first mentioned in the Black Chapters saga by Kurama. Eventually, the marriage occured and the married ones left for their honeymoon. Kazuya, Shiori and the two Shuichi live together in a big house.
He seems to get along well with his stepson, as in the manga Kurama said he didn't fear to leave his mother alone because she had Kazuya. Also, in the anime, Kurama refers to him as dad in a phone call he had with his mother.